Q:Is it possible to cast a three-unit bridge using an #8 gauge runner bar on the pontic without having porosity or a miscast?
A: An #8 gauge sprue will only work if the thickest part of the bridge is not as thick as the sprue, according to Patrick Naylor, DDS, MPH, MS. In his book, Metal Ceramic Technology (Quintessence Publishing), Naylor details The Fourth Law of Casting: "A reservoir must have sufficient molten alloy to accommodate the shrinkage that occurs within the restorations. Alloy that fills the restorations will solidify first. As that molten alloy solidifies, it shrinks and creates a vacuum. For a complete casting, the vacuum must be able to draw additional metal from an adjacent source--the reservoir. A runner (connector) bar can be an effective reservoir if it is equal to or greater than the thickest cross-sectional area of the wax pattern. The penalties for not obeying this law are: shrinkage porosity and/or suck-back porosity." When in doubt of which is thicker--the wax pattern or the runner bar--don't take any risks; add more wax to the runner bar or sprue to prevent porosity.
Answered by Dennis Ouye, CDT, February 2003
Q: I've heard of a technique to create clear margins on porcelain veneers or crowns. Is this really possible? If so, how is it done?
A: Clear margins for veneers can be done with refractory and foil techniques. Here's the procedure:
First apply the standard body or dentin colors. (Note: don't apply an opacified or fluorescent porcelain.)
Apply a more transparent layer.
Modify the transparent layer with the stains or modifiers in your porcelain kit. A photograph or drawing of a cross-section of a tooth found in most dental magazines is an excellent reference for how thick to build up each layer.
Clear margins for crowns are more difficult to fabricate because of all the factors of tooth reduction, type of shoulder preparation and metal coping design. Here's the recommended technique:
To eliminate the influence of the metal coping, cut back the metal to 2mm above the margin. However, margins of this design are very fragile and require more than adequate reduction for strength. You should also use an opaceous margin porcelain at the edge of the coping to cover the transition from metal to translucent margin; this opaceous layer must end short of the margin.
Once the edge of the metal is hidden, cover the rest of the margin with margin porcelain and translucent porcelain. Keep in mind that the opacity of margin porcelain varies from brand to brand and may need to be modified to create the desired effect.
Answered by Dennis Ouye, CDT, February 2003
Q: How do you design hollow pontics?
The purpose of hollow pontics is to reduce the amount of weight or mass of the metal pontics. They allow the porcelain to be under compression when baked and reduces fractures because the porcelain is not under tension as it is with typical pontic designs. To create hollow pontics, put holes in the occlusal and buccal surfaces of the pontic waxups to reduce the bulk of wax wherever there may be stress. The holes can be any shape or size depending on the size of the pontics, and even one hole is better than none. However, designing hollow pontics correctly can be extremely difficult, even for the most experienced technician, which is why I prefer to purchase hollow pontics in pre-waxed form.
Answered by Dennis Ouye, CDT, February 2003
Q: I'm looking for a better technique to fabricate porcelain margins? Can you help?
A: Techniques that use resin or wax as a medium to help control the margin porcelain have recently been published. However, to allow for the free exchange of sodium between the particles of porcelain, I prefer to use water as a medium. Using water allows more intimate contact of the ceramic particles and therefore a stronger porcelain margin. The margin will also appear more vital and esthetic.
Here's the procedure; it is challenging and a visual demonstration by someone familiar with the technique can be invaluable:
First, prepare the die by lightly coating it with a die hardener and lubricate it with a separator. The chemical composition of some separators can create problems such as softening the surface of the die hardener, so select the separator appropriately. For example, if you're using a resin-based die hardener, avoid an alcohol-based separator.
Mix the margin porcelain to a doughy consistency, apply it to the edge of the coping and gently push it to the edge of the die with a brush.
Lightly condense the porcelain by ultrasonic vibration, lightly tapping it with warm air using a dryer or placing it in front of your oven. The porcelain should still be wet but not appear to be wet on the surface.
Push the coping down a few times on the die, then slowly lift it off. The entire porcelain margin should be intact with a few specks of ceramic on the die, but not at the marginal area.
Apply the second layer of margin material using the same technique. Don't attempt to add porcelain to the underside first and push the coping on the die; you can fill the underside voids with a third bake if desired.
Answered by Dennis Ouye, CDT, February 2003
Q: Why does sandblasting the occlusal surface on a full coverage crown prevent porcelain lift-off on the second firing?
A: Sandblasting roughens the occlusal surface--or breaks the surface tension--so the porcelain will stick to it. Just as water rolls off the surface of a polished car, porcelain will not stick to a polished surface. Sandblasting is only necessary if the surface is polished or glazed.
Answered by Dennis Ouye, CDT, February 2003
Q: What type of wax can be mixed with porcelain to make corrections to margins?
While you can mix wax with porcelain to correct a margin, I recommend purchasing a porcelain margin material that has already been mixed with wax. These pre-made formulations are consistent and save time. However, if you decide to mix it on your own, use a wax that does not produce any ash while baking in the porcelain oven. There are several brands of ash-free wax available from suppliers and you should experiment to see which wax is best for you.
Most importantly, don't use a crown and bridge wax because it contains additives to enhance its flow and hardness at specific temperatures that usually produce some ash at porcelain firing temperatures.
Answered by Dennis Ouye, CDT, February 2003
Q: What is the best technique for dipping dies to produce bubble-free copings? Should you use margin wax?
I'm assuming that by margin wax, you mean a wax that is softer and draws from the die with a little forgiveness for undercuts. I prefer to use a harder wax for the margins because if there is an undercut at the margin, the wax cracks, indicating that there is a problem. A soft wax may draw and slightly distort without cracking, but by doing so, creates the potential for an open margin in the casting.
The following technique not only produces bubble-free copings, it also prevents voids from occurring in the fossa area of the prep:
Before dipping the coping, use wax to fill any major depressions in the preparation.
Dip the die into the wax at a 45-degree angle with the lingual cusps of the posterior prep entering the wax first. Use a smooth, fluid motion. Dipping too quickly or too slowly can create internal voids (probably the bubbles you describe) or horizontal linear voids around the internal aspect of the coping.
Continue dipping the die at this angle until the wax meets the tip of the buccal cusps.
Tilt the die at a 90-degree angle and dip the rest of it until the wax extends about 1 or 2mm beyond the margins.
Using a pointed waxing instrument, reheat the areas you filled with wax prior to dipping to ensure the two waxes have joined together seamlessly.
Answered by Bill Mrazek, CDT, May 2003
Q: How can I prevent chipping of die margins? What type of die hardener do you recommend?
A: Regardless of the type of die stone or hardener being used, the first thing to remember is not to create too much of an undercut under the margin when trimming the die. The more severe the undercut, the more likely the unsupported margin will chip. Try to trim the margins at a 90 o angle. This provides adequate support for the margin during waxing, finishing, porcelain butt application and polishing, and is a guide for a proper emergence profile of the restoration.
Also, make sure you're not mixing your die stone too wet. While it's tempting to make a more fluid mix in order to prevent the creation of air bubbles during pouring, this makes a weaker mix. Always follow the specific water/powder ratio recommended by the manufacturer.
I also recommend mixing the powder and liquid in a vacuum spatulator. Not only is it easier and more efficient than hand mixing, it removes bubbles more effectively, creating a denser and ultimately stronger mix when set.
In addition, try using one of the new, improved resin-like stones that already contain hardeners. They offer greater strength and resistance to chipping than conventional gypsum die stones. And last, but not least, epoxy is always an option. As with any material used for pouring models, be sure the setting expansion results in an accurate-fitting restoration.
If you decide to try a stone hardener, make sure it doesn't affect the setting expansion of the stone. The last thing you want to do is fix your margin-chipping problem, but create a new problem with your dentist-clients when your restorations don't seat like they used to at delivery.
Here's another tip: after marking the die margin with a red pencil, I use a good die sealer and hardener to paint a thin coat over the margin area. It prevents the red line from wearing off so quickly, seals the margin so porcelain butt margins lift off more easily (with the help of a separator) and helps keep the margins from chipping.
Answered by Bill Mrazek, CDT, May 2003
Q: What is the ideal vacuum needed to get smooth, bubble-free castings with phosphate investments? If my mixer falls below 27"Hg, I get small air bubbles, but I only need about 25"Hg for gypsum, die stone and plaster.
A: The ideal vacuum needed is indeed 27"Hg or better, if you can get it.
Here are some recommendations:
Make sure the filter in the mixer hose is clean and your vacuum investor is well maintained.
Check the following areas on your mixing bowl:
Inspect the seal on the lid to ensure it isn't worn or dried out, preventing you from getting a stronger vacuum. Also look at the top of the bowl for wear or chips in the plastic that also could cause a poor seal during mixing.
Check frequently for internal wear. Investment abrades the walls of the plastic bowl, allowing it to absorb liquid and alter the liquid-to-powder ratio.
Although some prefer to mix in a dry bowl, I like to keep my bowl filled with water when not in use. When I'm ready to use it, I pour the water out and lightly dry the bowl with compressed air until the plastic is just damp. This prevents the plastic from absorbing any more moisture.
Put your investment liquid in the bowl first, then pour the investment powder evenly in the bowl. Adding powder to liquid produces fewer bubbles. Then mix according to the manufacturer's instructions, keeping in mind that you may need to alter the liquid-to-powder ratio slightly to achieve the desired fit.
If you have enough working time, remove the bowl from the motor after normal mixing time, but leave the vacuum hose connected for an additional 15-30 seconds. This helps pull air bubbles from the investment as they rise to the surface.
In addition, this debubbilizer application technique works well:
Keep your debubbilizer liquid in a small Tupperware-type container.
Dip the waxups in the liquid after they're attached to the sprue base.
Using a squirt bottle, gently rinse the waxups with room-temperature water.
Dip the waxups in the debubbilizer liquid again and blow off the excess with a low-pressure air hose.
Place the ring on the sprue former and invest. The castings will be smooth.
Answered by Bill Mrazek, CDT, May 2003
Q: Sometimes the impressions we get from our dentists appear to have seams on the preps, forcing us to guess where the margin begins and ends. What causes these seams and how can they be prevented?
This is a common problem. Impression trays are often placed into the mouth at an angle and then lowered into position with a hinge-like movement. This causes the light-body material to be pushed laterally along the body of the tooth, creating a vertical void or seam in the impression material.
The seams usually begin near the occlusal surface (on posterior teeth) or near the incisal edge (on anterior teeth), and continue down and become broader near the gingival aspect. Unfortunately, many times the seams extend all the way to the gingival margin, making the margins on the impression "unreadable" and a new impression absolutely necessary.
To prevent seams, offer your dentists this impression-taking technique:
While the dental assistant is loading the impression tray with the heavy-body impression material, the dentist should simultaneously encapsulate the prepared teeth with light-body material. The remaining teeth in the arch can be covered with light-body material as well.
Next, keeping the impression tray containing the heavy-body material parallel to the arch, the impression tray should be inserted into the patient's mouth, then pushed straight down into position over the teeth.
By seating the impression tray at a 90-degree angle to the preparations, the light body is compressed and dispersed evenly around the tooth structure. It also places an even amount of pressure on the impression material around the margins, pushing the material into the sulcus, rather than pushing it laterally.If the dentist can't take a new impression, ask him to trim the die. He has a better memory of what that prep looked like and, this way, you're not responsible for misjudging the true margin.
Answered by Bill Mrazek, CDT, June/July 2003
Q: Do you have any recommendations for our dentist-clients to ensure accurate bite registrations and adequate occlusal reduction for posterior full coverage restorations?
A: A dentist-client and I have developed a technique that has literally eliminated posterior occlusal adjustments on our cases and I believe it will solve both of your problems. This technique provides solid centric and posterior stops for the patient during the bite registration procedure and is used before the patient is anesthetized to ensure a proper bite. It also allows the dentist to verify that the patient has bit properly, and to ensure that he has reduced the preparation sites for adequate occlusal clearance in centric as well as all excursive movements.
Here's the technique:
Before the patient is anesthetized, the dentist places a thin strip of articulating paper over the occlusal surfaces of the posterior teeth to be restored and asks the patient to bite down into centric occlusion. He uses the marks to identify the occlusal stop that is registered in the central fossa of the most posterior tooth that is going to be restored.
He dries the tooth and wipes off the mark, but marks the same spot with a fine-tip black permanent marker. The patient bites down again on red articulating paper. The red dot from the articulating paper should be directly on top of the black dot.
The patient is anesthetized.
The dentist reduces all of the posterior teeth to be restored, but he doesn't reduce the most posterior centric stop marked with the permanent marker.
He takes a double thickness of pink baseplate wax--which, when heated and folded over, is about 2-1/2mm thick--and has the patient bite down into the wax over the preparations. If the patient bit down into true centric, the bite registration will have a distinct hole in it at the point of contact in the central fossa. If a hole is not present, the patient did not bite down correctly and a new bite is taken while the patient is anesthetized.
Once the correct bite is verified, the dentist measures the thickness of the wax with a wax caliper around the contact point and in every area above the other prepared teeth. If the teeth are adequately reduced, the wax should measure at least 2mm in thickness in every area except for the perforation.
If necessary, he further reduces any areas where the wax measures less than 2mm and takes a new bite.
To ensure adequate clearance during excursive movements, he puts the bite back in place and asks the patient to move his mandible into excursive movements. He then remeasures to make sure the wax is no less than 2mm thick in any area. (Note: After some experience with this technique, the dentist can simply hold the bite up to the light to see if color variations indicate that any areas are too thin.)
A final bite registration is taken with a more stable material, again making sure there is a perforation in the bite at the remaining occlusal contact point. The dentist then reduces the occlusal island to represent a normal preparation, takes an impression, and sends the case with the final registration to the lab for mounting purposes.
Answered by Bill Mrazek, CDT, June/July 2003
Q: A crown was sent back to me be cause the color was off. The shade is C-4 and it is still too bright. What can I do to make it less bright?
A shade that is too bright--also described as 'high in value'--is a common problem with porcelain-fused-to-metal crowns and stems from the highly reflective surface of the opaque that covers the metal. The root of the problem is how technicians and dentists communicate shade. The selection of C-4 indicates that the shade has a very low value. However, we tend to focus on the chroma--or the intensity--of color that we see. To match the correct shade, select a value shade first followed by the hue and then chroma. There are shade guides on the market that allow you to select value separately from the hue. Also, computerized chroma meters are available for those who have difficulty discerning value.
To achieve the proper amount of grayness or value, you may have to remove a layer of C-4 body porcelain and replace it with an incisal layer or with a little gray mixture. In this scenario, I'm assuming there is enough reduction of the tooth to add the proper amount of porcelain to adjust the color.
Also, the C-4 body porcelain you're using may be a little high in value or may not be adequate to lower the value of the opaque beneath it. With some brands of porcelain, a layer of translucent porcelain on top may be the only layer needed to accomplish the appropriate value.
Answered by Dennis Ouye, CDT, January 2002
Q: What would be the best approach to repair a one-year-old crown where a part of the porcelain and opaque have chipped off?
The answer depends on the location of the crown. If the crown is in the posterior, a repair with composite or ceramic would not be strong enough and would break under function. In the anterior region, however, you can make a veneer and bond it to the deficient area. Be sure to trim off any rough areas or chips on the crown before taking an impression and, to fabricate the veneer, use a refractory model for an intimate fit. A well-fitting veneer relies less on the resin for strength and, therefore, the repair lasts longer.
Answered by Dennis Ouye, CDT, January 2002
Q: How do you make a "reduction coping" for the occlusal surface of a molar?
Reduction copings are a terrific way to accurately communicate the necessary reduction for an under-prepped tooth without having to call the patient back into the office for reprepping and a new impression. Of course, you should consult your dentist-client before you alter the preparation.
The following two techniques work very well. The main reason we use the first one--fabricating a non-precious reduction coping--is that the copings won't break if the dentist pushes down too hard during seating.
Here's the technique:
Make a wax coping on the original die that needs to be reduced.
Remove the coping and reduce the die. Mark the areas reduced on the die with a permanent marker.
Place the waxup back on the die. The marked areas should be easily visible through the .5mm of wax.
Carve the wax back over the reduced areas so the marked areas are exposed. The wax should end around these areas at the same angles as the reduction. This wax pattern is now the reduction coping and exhibits open areas on the occlusal surface wherever the coping was reduced.
Sprue and invest this pattern and cast it in non-precious alloy.
Wax a second coping as you normally would on the reduced die and cast it in your alloy of choice for the final restoration.
Send the reduction coping with the completed case and a written reminder that the prep has been altered. The dentist should seat the coping and reduce any tooth structure protruding through the occlusal surface of the casting, being careful to match the angles designated by the coping. Once this is accomplished, the restoration should seat accurately.
Another technique that's somewhat quicker is to make the reduction coping out of quick-cure resin or light-cured composite, which eliminates the need for waxing, casting and finishing.
Make a coping on the original preparation with one of the materials mentioned above. Be sure your resin or composite is thick enough to withstand finger pressure so it doesn't fracture.
Place the die back in the articulated master model. Then, using a handpiece with a stiff rubber wheel, simultaneously reduce the coping and tooth structure.
Once you've achieved adequate reduction, remove the coping and send it with the finished case.
Occasionally, the necessary reduction is confined to the central fossa area of the occlusal surface. The normal reduction coping techniques described above won't adequately communicate the reduction performed in the laboratory. Try this technique instead:
Reduce the fossa on the die as needed.
After the restoration is completed, paint separator on the occlusal surfaces of the teeth opposing the restoration while both models are still articulated.
Apply separator to the occlusal surface on the prep as well.
Using either self-cure resin or composite, make an occlusal bite registration between the preparation and the opposing teeth. Be sure the bite is completely closed and none of the resin gets into any undercuts--this could make removal of the bite registration difficult.
Extend the registration onto at least three of the opposing teeth.
Once the resin or composite has cured, open the articulation and remove the registration. Since the opposing model could accidentally chip while removing the registration, it's important not to make the registration until the restoration is completed.
Send this registration to the dentist. He should place it into position on the patient's teeth opposing the prep and then place thin articulating paper over the prep. The patient bites down lightly and registers marks identifying where the dentist needs to reduce. The dentist continues this procedure until the registration is completely seated and the patient's bite is closed; he then seats the restoration.
We charge a fee for this service and document on the original prescription that the dentist requested a reduction procedure and coping for the case.
Answered by Bill Mrazek,CDT, June/July 2002
Q: Should you separate all bridges interproximally before firing?
A: You can separate bridges interproximally and there are two ways to do it. Separating the buildup interproximally down to the opaque allows the porcelain to shrink without lifting away in the connector areas. However, this requires a second bake to fill in those areas.
Another option is to separate them interproximally without cutting all the way down to the opaque. In this case, you must be sure the porcelain has been condensed really well and the underlying framework is designed for a uniform thickness of porcelain so that shrinkage is minimal and even.
However, instead of separating bridges interproximally, I use a technique that eliminates both the possibility of hitting the opaque and the concern about additional shrinkage or tearing:
Apply a chroma modifier to the interproximals and the central fossas of the posterior teeth, as well as to the linguals of the anterior teeth.
Fire the bridge.
Do the rest of the buildup, cut back to the previously fired material interproximally, carve your occlusal anatomy in the posterior teeth and create lingual anatomy on the anterior teeth. If the case has porcelain butt margins, apply the chroma modifier at the same time you apply the second coat to your butt margin. This eliminates an additional firing.
Answered by Bill Mrazek,CDT, June/July 2002
Q: What is the best way to deal with marginal ridge height when mal-occlusion and super-erupted adjacent and opposing teeth are involved?
In these situations, I call the dentist to find out if the treatment plan includes any future work on the adjacent or opposing teeth. If so, I can then fabricate an ideal restoration without taking surrounding dentition into consideration; future restorations will be fabricated to improve the malocclusion and/or super eruption.
If future work is not planned, placement of the marginal ridges is determined by function, stability of interproximal contacts and contour/esthetics. The marginal ridges must be kept low enough so they don't cause lateral, protrusive or centric interferences. They must also provide adequate interproximal contact with the adjacent teeth.
Lastly, I determine what looks most natural. A marginal ridge that is too high compared to adjacent teeth may look as unnatural as one that is sloped down to create a ridge that is level with its neighbor.
Answered by Bill Mrazek,CDT, June/July 2002
Q: The porcelain has broken off the buccal of a pontic on a bridge. Does the technician have to redo the bridge or can he repair it?
A: A temporary repair can be done in composite and bonded to the cer amic, but it would only last for a few years. For longevity, a better repair is to cast a metal slip to put on top of the pontic. Try this technique:
To provide room for what is actually a porcelain-to-metal cast, remove a significant amount of porcelain from the occlusal of the pontic down to the metal. If there's room, cut a groove for retention and functional resistance into the metal of the occlusal surface without compromising the strength of the metal framework.
Prep the pontic, take an impression and follow the same procedures as for making a PFM.
After the technician completes the porcelain-fused-to-metal slip, bond it to the buccal-occlusal surface of the pontic.
To facilitate the bond to metal, sandblast both the underside of the metal slip and the metal surface of the pontic. Add micromechanical retention to both metal surfaces by impregnating glass to the surface by sandblasting with an aluminous glass bonding material, or tin plating followed by a Panavia-type luting material.
Answered by Dennis Ouye, CDT, January 2002
Q: Can a porcelain-fused-to-metal bridge be repaired after it's been in the mouth for a while? The bridge is loose and can be removed.
A: If the bridge has been in the mouth longer than a few months, it be comes difficult to repair. The longer the porcelain remains in the mouth, the more bacteria is stored in the porcelain. The porcelain must be decontaminated with a special solution and then slowly dried for several hours in a porcelain oven.
Even with all of this preparation, there may be some degradation of the ceramic from the acidic nature of the oral environment. However, this degradation isn't very visible until after porcelain is reapplied and the bridge is baked. It has the appearance of a white porous surface, sometimes with white or gray spots. The degraded surface must also be removed before any porcelain can be added on top of it. In the end, the repair is time consuming; removing all of the porcelain and replacing it with new porcelain provides a quicker, more esthetic result.
Answered by Dennis Ouye, CDT, January 2002
Q: I recently placed a crown with a porcelain margin over a metal post that looked dark at the margin once cemented. Is the darkness caused by the metal post or the dark shadow cast by the coping? Should the crown be redone and, if so, how can the technician eliminate the darkness at the margin? Is it necessary to remove the post?
A: There are a couple of reasons why some PFM crowns appear dark at the margins:
If the crown has an adequate porcelain margin--one that's at least 2mm in length--the darkness is most likely caused by the gray metal post showing through rather than the shadowing effect of the coping. In some cases, the metal is also visible through the tissue. My recommendation is to replace the post with a more tooth-colored material such as a ceramic, alumina-type post or a metal post that has been opaqued.
If the crown does not have an adequate porcelain margin--or none at all--this can block light from entering the tooth and create a shadow effect. Suddenly, a tooth that appeared lifelike and vital can look dull and gray at the margin.
If the crown needs to be redone, and the technician needs to extend the porcelain proximal to the mesial or distal more than 2mm, a PFM restoration is necessary for adequate support.
If the porcelain doesn't need to be extended in excess of 2mm, there are a few other options. For example, it can be replaced with an all-ceramic crown or a ceramic crown supported with a core. Although some ceramic cores can be reinforced with strengthened glass to provide more support, they still may not be as strong as metal. Pressable glasses, alumina cores or zirconia-type cores are other esthetic options. An all-feldspathic crown is also esthetic, but lacks strength compared to the other choices. Based on these parameters, the technician should choose the material he thinks will produce the best result.
Answered by Dennis Ouye, CDT, January 2002
Q: How do you achieve perfect occlusion? Which grinding and finishing methods would you suggest?
In order to achieve perfect occlusion, you need three things: a perfect impression, a perfect bite registration and a perfect temporary or provisional restoration. Stop laughing. This can be done and is done on a fairly regular basis. Obviously, in order for you to provide restorations that literally drop into place and function like natural dentition without adjustment, the dentist must do the best he possibly can to provide you with an accurate reproduction of what is happening inside that patient's mouth. For larger cases, a face-bow transfer is also advantageous.
Next, it is important to know what philosophy of occlusion the dentist practices. Does he want the restorations designed to create group function or anterior guidance with complete posterior disclusion? Also, does your dentist-client prefer a gnathological tricuspidization, flat centrum or landing pad occlusal contact? Since all of my clients follow anterior guidance, centrum contact occlusion (this seems to be the most common in the dental community), I will use this philosophy to answer your question.
I follow the Pankey philosophy that form follows function. In other words, teeth have to function properly, and their shape and anatomy is a result of--or at least in harmony with--that function. Therefore, I developed a buildup technique about 12 years ago that I call the Functional Buildup Technique:
Use stone sealer to seal the opposing occlusal surfaces, adjacent contact areas and saddle areas on the working model.
Once the stone sealer has dried, take a water-soluble, fine-tip marker and place a fine dot on the opposing cusps and fossas that are going to be in occlusal contact with the restoration.
Build the crown so that the buildup has normal contours; do not overbuild.
Carefully close the articulator so that the opposing model comes into contact with the buildup.
Open the articulator. The occlusal dots from the opposing teeth should transfer to the buildup. (Even though the dots are dry on the opposing model, the moisture in the buildup will cause the dots to transfer since the ink is water soluble.) These dots indicate where your final occlusal contacts need to be placed.
Close the articulator again, tapping lightly until the models are in complete contact. Keep a tissue against the buildup to absorb any moisture that may come to the surface. Do not leave teeth out of occlusion to compensate for shrinkage.
Next, open the articulator and move it into an excursive position. Gently tap the models back into centric occlusion. This exercise traces the excursive paths into the buildup. Do not start in centric and go into excursive unless you want to see your buildup on the benchtop. Always start out of occlusion in the movement and tap lightly back to centric. Do both left and right excursive movements as well as protrusive.
Once this is completed, open the articulator. There will be marked tracings where the opposing cusps have traveled.
Add a very small amount of porcelain to the dotted centric contact areas on the buildup, both in the central fossa and the working cusp tips.
Close the articulator again, but this time leave it open about 1/2mm. At this point, you are finished with the articulator.
Remove the restoration from the model and add to the interproximal contacts.
Carve your anatomy, but don't carve through or eliminate the dotted contact areas. This actually forces you to carve natural, curving anatomy. Think of the dots as little landmines. If you hit one, you've blown your occlusion!
Blot to absorb any moisture and fire as usual.
Upon seating, you'll find that, except for the contact areas, the entire occlusal surface has shrunk out of occlusion, eliminating any excursive interference. The only areas that may need adjustment should be the contact areas. You can quickly adjust the contacts in the fossa using a barrel diamond, leaving a flat landing pad or centrum for the opposing cusps. You will also need to slightly adjust the cusp tips. Grinding can be kept to a minimum by building to appropriate contours and not overbuilding. The average amount of time I spend adjusting a porcelain restoration before glazing is about three minutes. I try to avoid grinding as much as possible: it's dirty, time consuming, unhealthy, expensive and--most of all--unnecessary.
Answered by Bill Mrazek, CDT, January 2001
Q: What is the best method for glazing composites?
A: To be honest, I don't believe there is one. The glazes that I have seen are applied and cured to the surface. The result is esthetic, but eventually wears off. Also, keep in mind that the patient is going to get his teeth polished by the hygienist at a cleaning appointment. If the glaze hasn't worn off by then, it probably will if an abrasive prophy paste is used.
In my opinion, a dense composite buildup that has been highly polished looks more natural than an artificially glazed surface. Most of the newer composite systems on the market are designed to polish beautifully, and should be one of the features to look for if you're considering buying a system.
Answered by Bill Mrazek, CDT, January 2001
Q: How can I reduce or eliminate interproximal tears when building PFM bridges?
A: Interproximal tears are usually related to one (or a combination) of three things:
Understructure design. One of the most common causes of interproximal tearing is designing an understructure with too much space between units in the connector areas. Generally, the porcelain buildup is 1 to 1.5mm thick. If one area is thicker than another, it will undergo more shrinkage. Therefore, if the connector space is wide, there will be more porcelain in that area, creating greater shrinkage and tearing. The connector space should not be wider than 3 to 3.5mm. This allows for 1.5mm of porcelain on the interproximal of the adjacent units and uniform shrinkage.
Too much moisture. The more moisture, the more air created when the water evaporates. This means a greater number of voids between particles, resulting in a greater degree of shrinkage when the porcelain is fired, creating tears. Be sure to blot as you build.
Bulky buildup. Do not severely overbuild to compensate for shrinkage.
Contour the restorations so that the transition from the facial and lingual surfaces into the interproximals is rounded.
After using a blade to cut interproximally to separate the units before firing, round off the corners to eliminate the excess bulk of porcelain. Otherwise, the shrinkage in these areas moves in a diagonal direction from the corner toward the understructure. This also creates tearing.
Here's another tip.
Layer some porcelain (higher chroma dentin powders, for example) in the interproximals and fire before doing any other part of the buildup. (If the bridge has porcelain butt margins, do this step at the same time you're applying the second butt margin application.) Both areas can then be baked simultaneously.
Build the rest of the bridge.
When you cut the interproximals with a blade, cut all the way down to the previously fired interproximal layer. This way, there is less bulk of unfired porcelain, eliminating tearing and you don't have to worry about hitting the opaque.
Answered by Bill Mrazek, CDT, January 2001
Q: My lab is having problems with noble alloy contamination. What could be causing the contamination and how can I prevent it? Could our spruing method be the cause?
A: There are several possibilities that could cause contamination of ceramic metals, especially noble alloys that contain palladium. While it's unlikely your spruing method is a consistent cause of contamination, sprues that are too long and thin could cause porosity if the wax patterns are not placed in the "cool zone" of the casting ring. Place wax patterns 3/8" to 1/4" from the top of the ring, away from the center of the ring's "heat zone" where there is a larger mass of investment to cool off. This allows the copings to cool first and the porosity (we think of it as gas) moves to the area that is still fluid. Reservoirs or runner bars that are placed in the heat zone create an "easement area" in which gas or porosity can form--rather than in the copings or cast crowns.
If you make this adjustment and still have problems, another source is probably the culprit. Here are some of the possible causes:
Airborne dust from metal grinding is the most common source of contamination in the laboratory. Metals and abrasive dust contaminate the wax and, since they don't burnout, they show up as porosity in the margin areas of the casting.
Buttons and castings that are remelted can appear to be clean but may still have small grains of investment on them. If any investment is melted with the metal, it creates gas and bubbles in the porcelain. Sandblast your buttons and castings and check them under a microscope to ensure that they're thoroughly clean. We sandblast with aluminum oxide and then place the buttons in 7% hydrofluoric acid.
If sprue formers (ring holders) are not cleaned thoroughly, small pieces of investment may break off their surfaces during casting and be carried into the flow of metal, resulting in porosity.
During casting, metal that flows through the rough edges of wax patterns and sprues can cause investment to break off.
Carbon residue from incomplete burnout of wax patterns or carbon-containing investments also causes bubbling in the opaque or bonding problems. An improperly adjusted torch with an overly reducing flame (too much gas) also introduces carbon into a melted metal. When palladium is exposed to carbon, contaminants can form in-between the grain boundaries of the metal. It not only creates gassing of the porcelain, but also affects metal expansion. Once the metal is contaminated, the buttons should be scrapped and new metal used for future castings.
Resin binderfrom stones or discs can attach to the surface of a coping while grinding and cause contamination on the surface of the casting. (You can tell if stones or discs have resin binders from the burning plastic odor they emit during grinding.) Degassing the coping under vacuum for 10 minutes will turn the resin to ash and dissipate the ash from the coping.
Answered by Dennis Ouye, CDT, February 2001
Q: I've been experiencing porcelain cracking caused by a pancake or flat bubble about 6-8mm in diameter that lifts the opaque from the metal coping. We're processing 2% gold palladium alloy the same way we process base alloys and have never had a problem with the base alloy restorations. At the advice of the manufacturer, I've been using a carbon-free investment. I also use the same finishing stones on all of my alloys.
A: Porcelain cracking or flat bubbles in the opaque area can be caused by your spruing method, airborne dust from metal grinding, remelted buttons and castings, improperly cleaned sprue formers, casting and carbon residue or resin binder. In addition, using the same finishing stones on all of your alloys could be the problem. High noble alloys require not only a separate stone or burr, but one that is different than the ones used for base alloys (Metal Ceramics, Makoto Yamamoto, Quintessence Publishing, 1985). High noble alloys tend to roll, fold or burnish more than base metals, which can create gas pockets. When you use a stone on a base alloy, metal can attach to the surface of a stone, then, when you grind a noble or high noble alloy, the surface will be contaminated with the base alloy debris.
Answered by Dennis Ouye, CDT, February 2001
Q: All the ceramists in my laboratory are having a problem with tiny bubbles that come to the surface after the glaze bake. They are located circumferentially around the PFM crown in the gingival area.
A: Bubbles in the porcelain are entirely different from bubbles that are related to metal contamination, which expose the metal underneath. Here are some possible causes:
When wet porcelain is added to dry porcelain, air becomes trapped in the lower layer and eventually appears and rises to the surface with subsequent bakes. The problem becomes more apparent when layering porcelain on a refractory model because of the constant wicking from the refractory material. It's important to keep the porcelain wet at all times; if it dries, the porcelain has to be rewet and vibrated in an attempt to rewet all of the particles. Modeling liquid can be added to porcelain for a longer working time and is very helpful, especially if several colors are used.
However, if bubbles still appear on the surface, it's best to grind the porcelain down to the opaque layer and restack or fill the bubbles with low-fusing porcelain if necessary. When low-fusing porcelain is baked in small amounts to approximately 820 degrees, it will appear glazed over and is low enough in temperature to prevent more bubbles rising to the surface.
Contamination of the porcelain furnace. If refractory investment is degassed in the porcelain furnace enough times, the porcelain oven may have difficulty pulling full vacuum because chemicals exfoliate from the investment and attack the seals of the oven. Without complete vacuum at the critical time of porcelain particles fusing together, thorough vitrification is inhibited and air bubbles may be present in the deeper layers. I recommend burning out refractory models in a burnout oven prior to degassing in a porcelain oven. Also, purging the oven will ensure that there is no contamination.
Answered by Dennis Ouye, CDT, February 2001
Q: On 40% of my bridges, the metal frame seems to rock. I'm certain that I must be doing something wrong and hope you can give me some advice. Here's my technique:
I use die lube and wax dip the prepared dies. Using an electric spatula, I add yellow margin wax to enhance the margins of all dies on the bridge. After it sets for 15 minutes, I examine the dies under a 10x microscope and scrape off the excess wax. Once I achieve the desired thickness, I cut and shave the wax margins as perfectly as possible. I seat the dies on the model and connect one die to another with a #8 firm, solid plastic rod and rigid, brittle dark brown sticky wax. I begin from left to right, waxing mesially only, then from right to left connecting distally. I let it set for 10 minutes, build up the pontics with carving wax, and let it set again for 10 minutes.
For each die, I either use a straight bar with individual sprues or a V-type sprue with individual sprues. In both cases, I use the same rods and wax that I use for connecting. I let it set for 10 minutes before I pull the wax framework from the model; in most cases, due to the angle of preps, some dies come off the model. I then pull those hanging dies separately.
A: You show great attention to detail in your technique. However, I do have some suggestions.
For connectors and sprues, switch from the solid plastic rods to hollow plastic rods. As the bridge pattern burns out in the ring, your wax copings melt before the solid plastic rods. As the wax vaporizes, pressure from the liquid wax and resulting gases build up within the ring. Since the solid plastic rods have not begun to melt, the pressure built up inside the ring can cause cracking in the investment that results in fins in the casting, or worse, the investment at the top of the ring can literally be blown off. Any time cracking in the investment occurs, warped bridges can result. Hollow rods allow the wax and gases to pass through like a straw, therefore preventing any build up of pressure.
I notice that you attach your bridge abutments together with the plastic rods before building the pontics. I suspect that one of the reasons your bridges are rocking is that as you flow wax in the pontic areas, the sticky wax that is holding the plastic rods in position may become heated in the process and distort the relationship of the two abutments. Try attaching just one end of the rod to an abutment, build the pontic and then attach the other end to the remaining abutment.
Also, sticky wax has a tendency to expand and contract. For your connections, I suggest switching to the wax you use for the rest of the waxup.
Here's another alternative: once the bridge is totally waxed, use 1/2-pound fishing line to separate the abutments and pontics so each die doesn't connect to another die abutment section. Then build an appropriate sprue system on the bridge and let it set. Reconnect the abutments and pontics with dead wax using as little heat as possible. Let it set, pull patterns and invest immediately.
Instead of allowing so much setting time between procedures, move immediately from one step to the next. I realize that your intention is to correct any distortion before moving on to the next step, but this actually allows more time for distortion to occur.
If you are using a fast burnout investment, make sure you follow the manufacturer's recommendation for putting the ring in the preheated oven. For proper expansion, it's imperative that the ring is not allowed to bench set too long. An inexpensive mechanical timer with a bell solves this problem easily and can be heard over other laboratory noises (like grinding, trimming, suction units, snoring, etc.).
Wax frameworks can definitely be distorted while being removed, especially those with non-parallel abutments. On bridge cases--especially large ones--I survey all the abutments if I suspect there may be a parallelism problem. If a problem exists, call the dentist and ask if the non-parallel preps are vital or if the patient has had root canal therapy. If the preps are non-vital and your dentist-client agrees, alter the preps to provide the necessary parallelism, and provide reduction copings so that he can accurately duplicate your reduction chairside. If the preps are vital, the dentist may want to see the patient again to determine if more reduction is possible, or if the case needs to be redesigned. Identifying this problem up front saves both you and your dentist-client time and frustration.
Answered by Bill Mrazek, CDT, October 2001
Q: Some doctors prefer small pontics on posterior bridges and others prefer them to be the normal size. How do you determine the size of molars as a pontic on a bridge?
A: Some doctors prefer small pontics bucco-lingually because they reduce occlusal forces on the pontics and on the abutments that support them. There are a number of factors I consider with posterior pontic size, including the number of pontics between the abutments (span length), the number of abutments at each end of the bridge (single- or double-abutted, etc.), the periodontal condition of the abutment, and the width of the occlusal tables of the adjacent and opposing dentition.
From a cosmetic standpoint, I prefer to make the pontics the same width as the natural teeth they are replacing. However, from a functional standpoint, if the bridge has three pontics between abutments, for example, I generally reduce the bucco-lingual dimension by about 25-30%. I try to make the transition from the anterior abutment to the posterior abutment a gradual one. In other words, the first bicuspid pontic is no wider than the distal of the anterior cuspid supporting it. The mesial of the second bicuspid is the same width as the distal of the first bicuspid, but slightly wider on the distal. The mesial of the first molar is no wider than the distal of the second bicuspid, with the distal of the first molar being, again, slightly narrower than the mesial of the normal-looking posterior abutment. In this manner, the reduced dimensions are not as apparent visually or to the patient's tongue. If the bridge has good periodontal support or multiple abutments, I design the pontics with normal contours.
Answered by Bill Mrazek, CDT, October 2001
Q: I'm having trouble casting molar pontics without surface porosity. Any suggestions?
Surface porosity occurs because the amount of alloy needed for the pontic can easily be four times the amount needed for a coping abutment. Therefore, before cooling takes place, the alloy must fill the pontic and the abutment at about the same rate. The pontic cools at a slower rate than the abutments due to its larger mass. As it cools, it contracts and needs to draw more alloy from the sprue. If the reservoir and sprue cool before the pontic, there is no place from which to draw additional alloy. Porosity then results in the area where the sprue is attached, on the surface of the pontic, or both.
Double spruing every molar pontic has solved this problem in our laboratory. Using two sprues, one for each of the buccal cusps (or lingual cusps, if spruing from that direction), helps the alloy flow into the pontic more efficiently than one large sprue. Also, each of the sprues has its own reservoir from which to draw.
Also, be sure you're following these basic rules:
Don't overwind your casting machine. Excessive centrifugal casting force can cause turbulence in the alloy as it fills the pattern, also resulting in porosity.
Be sure your wax patterns are no more than 1/4" from the top of the ring. This allows gases to escape during burnout.
Answered by Bill Mrazek, CDT, October 2001
Q: I'm using a pressable ceramic to fabricate onlays and inlays. Why are they cracking at the divesting stage?
A: I suspect that the cracking you're experiencing is actually happening before you divest, rather than being the result of divesting. When experiencing a problem with pressable ceramics, it's important to go through a checklist to identify the cause:
Make sure your pressing oven is correctly calibrated and you're following the pressing parameters recommended by the ceramic pellet manufacturer.
Make sure your investment is designed for pressable ceramics and for the type of pellets you're using.
Attach the sprue to the thickest area of the inlay or onlay. This is usually at a 45 o angle on the marginal ridge. If the restoration is a large MOD inlay or onlay on a molar, for example, attach a sprue to each of the marginal ridges. Position the sprues at an angle where they meet in an upside down V-shape above the occlusal surface.
Be sure the sprue diameter is similar to--but not greater than--the area to which it's being attached. Cracking can by caused by different cooling rates in the ceramic. I have found that if the diameter of the sprue is too wide, cracking can occur at the sprue attachment site. Attach the sprues at about a 15 o angle from the sprue base, with the patterns angled toward the center of the ring.
When the pressing cycle is over, immediately remove the rings from the pressing furnace by grasping the rings at the top, not in the middle. The room-temperature tongs can create thermal shock in the center of the hot ring where the patterns are located. (It's similar to how it feels when the doctor puts the 'room-temperature' stethoscope on your chest.)
Lastly, make sure the inlay and onlay preparations don't have any sharp internal angles. If they do, try to round these internal sharp areas off slightly during blockout and in the wax before investing.
Answered by Bill Mrazek, CDT, November/December 2001
Q: I'm getting check lines on pontics 8, 9, and 10 when the case is finished. Why?
A: There are two common causes of cracking in pontics:
Framework design. On anterior pontics, the waxup should mimic the size of a normal prep with no sharp angles or edges present. Pontics are often under-waxed to reduce metal cost or for fear of porosity. However, it's important that the porcelain has adequate support, especially when the patient goes into a protrusive movement and the upper and lower incisors approach an end-to-end bite. Also, if there is an excessive amount of porcelain at the incisal edge, the porcelain will cool at two different rates; the material in the middle third of the pontic cools more slowly than that at the incisal edge. This can cause a horizontal or diagonal check line after the case has cooled. This can also occur on the gingival aspect if there is an excessive amount of porcelain under the pontic.
Incompatible coefficients of thermal expansion (CTE) between porcelain and alloy. Porcelain adheres to metal in three ways: (1) a chemical bond in which the opaque ceramic bonds to the oxide layer of the alloy, (2) a mechanical bond that is the result of the opaque flowing into the variety of grooves and textures created in the surface of the alloy by metal finishing, and (3) a compressive bond created by an acceptable mismatch in the coefficient of expansions of the porcelain and the alloy during heating and cooling.
Within this mismatch, there is a range of compatibility that allows the use of alloys with different CTEs. The CTE of the alloy is usually printed on the bottle label or in the instruction manual. It's important to be aware of the CTE because metal holds heat longer than porcelain and therefore shrinks at a different rate. Therefore, the less compatible the numbers are, the greater the chance of checking. This is especially apparent on large pontics, which cool at a slower rate than the abutments.
Ideally, the CTE of your porcelain should be one point lower than the CTE of your alloy. For example, the porcelain I use has a CTE of 13.1 and my alloy has a CTE of 14.1, a difference of 1.0. The ideal alloy CTE for my porcelain is 14.1. The acceptable range of alloy CTE is +/-0.5 of 14.1, or between 13.6 and 14.6.
If the CTEs of the porcelain and alloy are compatible and the understructure offers the proper design and support, checking can be prevented by varying the cooling rate after firing--based upon where the alloy's CTE falls within the acceptable range of the porcelain. The reason for varying the cooling rates is to get the porcelain past the critical point when checking occurs in a manner that compensates for the differences in the CTE of the two materials.
If the CTE of the alloy is at the low end of the acceptable range, between 13.6-13.9, the porcelain should be fast cooled: the platform comes straight down out of the muffle and the work is moved away from the heat source. If the alloy CTE is between 14.0-14.3, I use a normal three-minute cool cycle. If the CTE is between 14.4-14.6, I use a six-minute cool time.
Answered by Bill Mrazek, CDT, November/December 2001
Q: I'm having small tears and pinhole bubbles in my porcelain and it's lifting away from the opaque at the margins. Any hints?
A: It sounds like you still have mois ture in your buildup before you fire the porcelain. Tearing is usually the result of moisture trapped below the dried surface of the buildup. When the porcelain is heated and the moisture evaporates, it escapes by literally creating a tear through the material as the vacuum pulls out the water vapor. Tears usually occur interproximally, where it is most difficult to blot efficiently. The pinhole bubbles are probably occurring for the same reason. Here are my recommendations:
Blot your buildup more frequently and keep your porcelain mix less wet on your palette. If you need to add additional porcelain to a buildup that has dried, spray it with a fine water mist from a pump spray bottle so the buildup is remoistened uniformly. Add porcelain as necessary and reblot entire restoration before firing.
Increase your dry time.
If you're using a buildup liquid, try cutting its concentration by adding distilled water to create a 50/50 mix. Buildup liquids are designed to create longer working times, but this also creates longer drying times. By reducing the concentration of the liquid and blotting a little more efficiently, your tearing and bubble problems should disappear.
The problem of the porcelain lifting away at the margins is also related to moisture and the volume of porcelain you have built at the margin. The greater the volume of material applied, the more moisture there is, and therefore, the greater the shrinkage. Here's my advice:
Don't overbuild the margins. Use a soft whipping brush to bring the porcelain down to the contour you want on the finished case. There will be less shrinkage and you won't have to grind near the margin after the case is fired.
If you clean the inside of the coping with a wet brush before firing, be careful not to inadvertently rewet the porcelain at the margin. Rewetting causes shrinkage to occur as if it had never been blotted at all.
Answered by Bill Mrazek, CDT, November/December 2001
Q: What are some causes of porosity? Can you give me some hints on prevention?
A: Porosity in a metal-ceramic restoration originates in the metal substructure, opaque layer and/or body and enamel layer. To ensure that a finished ceramic restoration is porosity free, careful observation during each stage of fabrication and attention to detail, from waxing to final glaze, is imperative.
- Metal substructure. Of the three areas, the metal substructure is most likely to be at fault because there are so many variables involved. The type of alloy dictates the techniques that you should use in each stage. For example, wax pattern thickness, sprue length, gauge and placement, investments and casting temperatures vary depending on whether you are using a base metal, noble or high noble alloy. Most manufacturers stipulate requirements for each step and you should follow their recommendations religiously.
Some causes of porosity in the metal substructure are:
**Reusing buttons and sprues:* this can introduce contaminants into the substructure and/or alter the formula of the original alloy, both of which can create an ideal environment for porosity. Reusing base metal almost always causes porosity because it requires a higher casting temperature than noble and high noble alloys. This high temperature has a tendency to deplete certain critical ingredients in the alloy of the original casting, thus increasing the possibility of altering the formula. Since base metal is relatively inexpensive, these buttons and sprues should be discarded.
On the other hand, the melting temperatures of noble and high noble alloy are somewhat lower so there is less of a chance that the addition of buttons or sprues will change the formula of the alloy. As long as the percentage of reused noble and high noble buttons and sprues is 10% or less of the total alloy in the melt (though some manufacturers recommend up to 50%) and they are cleaned well with an aluminum oxide blaster and steam cleaned or rinsed by ultrasonic with distilled water, the possibility of porosity is negligible. A good rule to follow: discard base metal buttons and sprues and use caution when reusing noble and high noble alloys.
Spruing, venting and investing. Sprue size and location in the casting ring, proper venting and molten metal reservoirs are important. Follow this technique:
Feed 10-gauge sprues into an eight-gauge runner bar that acts as a reservoir.
Place a 1/4" eight-gauge sprue on the runner bar to form an indirect flow to each unit. The runner bar should be located in the hottest spot of the mold to ensure that molten metal is available for each unit. The indirect sprue location permits an even flow of molten metal, preventing suck back that could cause porosity.
The margin of each unit should be 1/4" from the end of the ring with a 12-gauge vent. This allows gases to escape, preventing porosity as well as shy margins.
Use only the investment prescribed by the alloy manufacturer, and precisely follow directions pertaining to powder/liquid ratio, spatulation and setting times, etc. Proper investment and investing technique also permits gases to escape.
Casting. Assuming the wax patterns have been correctly sprued and invested, the next step is to establish a casting technique that eliminates the possibility of porosity.
Of the two casting methods--automatic and manual--the manual (torch) is probably the most critical. As long as the temperature and timing are correctly calibrated in the automatic casting system, the casting should be flawless. However, casting with a torch involves several variables that could result in a porous substructure.
Bottled gas (propane) is a better melting source than natural gas. Propane is clean, while natural gas contains impurities that can be forced into the substructure. When these impurities burn off during the porcelain firing stage, porosity results. An accurate gas/oxygen ratio is equally important; excessive oxygen in the mix causes porosity.
Overheating metal during casting can burn off trace elements that help prevent a porous substructure; underheating doesn't permit the mold to fill completely before solidifying, causing massive internal porosity.
Since base metal is lighter in weight (specific gravity measurement) than noble and high noble alloy, more force is required during casting to produce a dense substructure. Several extra turns on the casting machine--manual or automatic--accomplishes this. The number of turns depends on your casting machine and the manufacturer's recommendations.
Dressing down the cast substructure: this should be the final step before porcelain application. The biggest concern is the possibility of grinding contaminants into the surface of the metal. Heatless stones and others like them--ones that have a tendency to flake off abrasive particles--are especially troublesome. Only use carbides, diamonds and stones prescribed for metal dress down.
Since base metal is hard and its grindings flake off, there is less chance of surface contamination. However, when you are dressing down a much softer high noble substructure, even with the most appropriate stones and diamonds, the grindings don't flake off as readily. Therefore, there is danger of microscopic surface rollover that can trap air and gases (that result from burned off particles in the alloy) that can be expelled during porcelain firing.
The last step of
