In part two of this article, Boris Kovachev shows what can be done with an affordable ceramic powder that is ideal for these trying times.
What happened next
What happened next is roughly a mirror image of the enamel layering. I applied Dentin A3 to finish the cervical contour and dental equator. Then I extended the Dentin to the incisal area. This reproduced the initial contour of the aproximal vestibular enamel edges. Again we are taking advantage of the reversed layering, when covering the transparent horizontal belt with dentin.
Thus, enclosing light and achieving the all important de-saturation and neutrality of the middle third. The aproximal vestibular enamel edges, when made out of dentin, with more transparent enamel underneath, will be brighter and more pronounced under certain light conditions, with increased natural opalescence. While at the same time still exhibiting the Dentin A3 color.
The slightly concave area of the incisal third that’s left between the Dentin A3 aproximal ridges was filled with a mixture of Enamel Light and Transparent (1:1), while some Enamel was layered unevenly on separate spots over the root, for uneven scattering of light and added characterisation (Figure 7), where Transparent is died dark-blue, Enamel is blue and the Mixture of Enamel+Transparent is white).
In the same time minor portions of undiluted Transparent have been applied just below the cervical transition crown-root. This adds visual contrast and further separation.
We are well aware that “Enamel” and “Dentin” are just names of the porcelain materials. They aim to reproduce the appearance of the corresponding dental tissues of the human dentition. At their thickest and most pronounced. It is therefore entirely up to us where in the crown and for what purpose we choose to use those masses.
There are no said rules
There are, however, the laws of physics and optics which have existed long before the birth of dental technology. These tend to hold more ground than the misconceptions of strict ceramic layering. For example, the natural enamel covers the entire natural anatomic crown. So, after the first firing we shall be forced to take steps and reproduce the added translucency in the cervical area that comes from the thin enamel in the cervical third.
The Enamel porcelain however is just a material with set colour and opacity. It is not to be used in its pure form to cover the entire tooth, unless modified with another material. Otherwise we might accidentally stray from the desired shade.
Then again, as we just said – porcelain Enamel is but a material-possesses therefore certain shade, brightness and opacity – hence we can use that material freely everywhere we might need those exact characteristics. Combine that with the possibility of adding opalescence by using that particular set of properties over a more translucent mass (the sandwich technique in reverse layering). You’ll find yourself able to control 100% of the appearance of every millimetre of your restoration.
Fixing properly
Next I gently lift the wet handkerchief off the towel, with the finished vestibular layering of the tooth. I turn it over on a dry piece of absorbent paper (ceramic build-up facing down). It’s allowed to dry for couple of minutes and then the wet hankie is gently pealed off.
What we have before us is the uneven and defect ridden dorsal side of our layering. There are always unassembled parts of porcelain and bubbles there that we need to gently but properly fix before firing.
The easiest way to do that is to simply use the wet tip of the brush and vibrate the defect (without directly pushing ceramics in), until it closes under the flow of moisture. If then some sections of the back surface should become too concave I simply fill them with some dentin or corresponding material. During those manipulations the finished vestibular surface of the build-up is lying on top a dry paper towel.
It’s unlikely for it to excessively bend or tear, provided we work carefully. At this stage we could simply continue with the lingual layering over the unfired dorsal surface, but I personally find it more comfortable and reassuring to perform a first stabilised firing of the Vestibular “half”, prior to continuing. After a 10-15 minutes setting on the dry paper, we position the half of our sculpture on firing wool and commence first bake.
Metal free prostheses
In such metal free works as well, as in long-spam bridges with lots of pontic units, the end-temperature should be increased. The reason for that is quite simple, and again dictated by the laws of physics. Metal is a good conductor, hence the metal framework of a pfm restoration provides ample heating, firing and stable shrinkage of the ceramic from the inside. The temperature cycles, provided by the manufacturer are intended for firing on metal.
Porcelain on the other hand is a very good isolator and conducts heat poorly. So when performing metal free restorations, thick fully anatomic representations (as is the case here), large spam bridges, or any other case when added thickness and volume of ceramic is achieved, the material itself prevents the inner layers from developing sufficient temperature, needed for firing and the “melting together” of all particles is compromised.
Ceramic shrinkage
The increase in temperature is entirely up to the individual expertise and experience of every technician. It depends also on the type, quality and working condition of the ceramic furnace being used. To my opinion, when performing such anatomic representations, an increase of 20ºC -25ºC is quite sufficient. But I also add a firing time of one-two minutes at the final temperature of 945ºC. The drying time is almost doubled.
The increase of layered ceramic thickness will always result in greater shrinkage after firing – again the laws of physics. Because metal is such a good heat-conductor, when the framework is absent, the ceramic shrinkage after firing will be far greater than with the same porcelain on a pfm-case.
So the combination of both – thickness and metal-free structure – will always give a serious shrinkage in volume and size. Keep that in mind when performing fully anatomic representations of teeth, as well as with metal-free veneers on refractory dies.
After firing the vestibular half, we check for defects and if any are present we trim/grind them away with rotary tools. After that I used only Dentin A3 to build up most of the lingual side of the central incisor, with minor details on the root-edges layered with Enamel (Figure 8).
Layering again
Next I layered again undiluted Enamel, reproducing the lingual anatomic characteristics and enamel ridges. The choice of Enamel material here is due to its light colour and increased brightness. This makes it suitable for the reproduction of lingual enamel structures (Figure 9). After that I turned the newly applied material down, so it would face the dry paper towel. I started my final ceramic application to the vestibular surface of the crown.
As we said – in the natural dentition the entire anatomic crown is covered with enamel. So, we must represent that overall superficial translucency, without producing the glassy look of the older generation of materials. After all, in nature nothing is as clearly transparent as glass, so we mustn’t allow glass in the mouth of our patients. For such purposes of superficial layering the Enamel+Transparent (1:1) mixture works very nice, without being too transparent, or too dominant with the pure Enamel shade.
I began the vestibular build-up by first securing the vestibular aproximal enamel ridges. Over the already existing sandwich structure in those areas I applied a thin layer of Dentin A3 to secure the effect I was aiming for – vital appearance of a healthy and thick enamel, that would define the optical presence of the vestibular surface borders.
At the same time this would be providing us with some additional shade characteristics with the “shining” Dentin A3, whose opalescence we had effectively increased (Figure 10).
The sandwich technique
Next I started covering the entire vestibular surface with a mixture of Enamel Light and Transparent materials, in even portions. For this particular tooth I decided to use this superficial layer to imitate some enamel cracks. Since we were already fully capable to use multiple-material optical layers for every other effect, I chose to use similar approach with the cracks. First I applied the Enamel+Transparent mixture to the point where I needed my crack to appear.
Next, that very same mixture was used to build up a wall, from the cutting edge to cervical. This followed the path of my future crack imitation. It was perpendicular to the already fired porcelain surface through its entire length (Figure 10).
Again the sandwich technique would aid me considerably – I applied a very thin layer of Dentin A3 on the entire vertical wall of Enamel+Transparent and tried to smooth it out so the thickness of Dentin would gradually come to almost 0 at the newly formed vestibular surface (Figure 11). This way the light that passed so freely through the semi-transparent Enamel+Transparent mixture would crash in the wall of Dentin A3 and a crack-like formation would visually appear.
The same situation in the aproximal areas of the vestibular surface will not result such rapid crack-like formations, since there we have achieved a smooth layer transition between the Dentin A3 and the Enamel+Transparent mixture. It becomes visible that the appearance of the sandwich layering depends not only on the contrasting materials being used, but also on the manner in which the layers overlap.
Variation on the classic sandwich technique
This variation on the sandwich technique for crack-imitation was shown to me by a very talented Swiss dental technician. So, it is not a personal discovery of mine. He used for the same purposes, however, more contrasting materials in terms of their opacity. Which looking for a more discrete and unobtrusive appearance, this is unnecessary. The more contrasting the opacity of the “crashing” materials at their transition or border – the more pronounced and visible the effect will be.
The modern feldspar materials now also have enough embedded translucency. Enough to make the usage of the sandwich technique of reverse layering quite usable on all depths. Also in all material combinations, without the risk of spot/blob-like results.
We know this well from the older generations of ceramics. That is why I chose to use portions of undiluted Transparent around the cervical transition on the porcelain root. To achieve optical contrast and separation from the “anatomic” crown.
Afterwards I finished the crack representation by enclosing the more opacious material Dentin A3 with the Enamel+Transparent mixture, so as only a very thin line of the Dentin would remain bare on the vestibular surface. I extended the layer of semi-translucent mixture to the designated location of my second crack, and since I had decided to make that one shorter, only to the middle of the crown, I formed the defining wall and the stopping surface for it as well.
Thus I would make the crack only as long as I needed it to be, without any doubts or worries (Figure 12). The second crack was finished in the already described manner, with subsequent smooth transition from the semi-transparent mixture to the Dentin “reinforcement” of the medial vestibular enamel ridge (Figure 13).
Incisal Halo
As you can see on the last picture small portions of Dentin A3 completed the vestibular shape to the cervical border, thus when meeting with the undiluted Transparent, I had layered there prior to first firing, additional visual separation would be achieved.
In the incisal margin I chose to represent a youthful Incisal Halo. The easiest way to do that is again in keeping with the sandwich technique. We simply had to apply a thin string of Dentin A3 on the vestibular incisal end. We had to smooth it down so it would blend with the vestibular surface. To the lingual it would be left unconnected. This provides a very natural pattern of abrasion, which is present even in teenage patients.
The light that passes freely through the Enamel and surrounding materials would crash with the Dentin A3 halo. It would make it shine from within (Figure 13). We then fire on the same increased final temperature as the first firing. However, with decreased holding time during firing, or decrease the firing temperature by 5ºC -10ºC.
The incisal halo occurs in young well mineralised enamel incisal margins. The thickness is small and thus light gets scattered and transmitted simultaneously from multiple directions.
The effect in porcelain restorations can be achieved in various ways:
- Trimming the incisal margin at 45º towards the lingual surface, might limit the possibility to make an uneven and natural looking contour of the cutting edge itself;
- Applying specific superficial stain over the incisal margin, would demonstrate too consistent of an appearance and will not vary under differing light conditions and view angles
- By applying Dentin of the base shade for the said restoration in a thin outline over the incisal margin – this will provide increased opalescence and the closest possible appearance to the natural teeth, which under transmitted light present a halo with the same basic shade as of the observed tooth. After layering the Dentin string on the incisal margin, we only need connect it to the vestibular surface, the medio- and disto- lingual edges. The free-standing lingual side of the Dentin halo will outline a natural looking abrasion imitation.
Canine tooth porcelain build-up
For my next porcelain build-up I chose a canine tooth – those are quite massive and because of that solid volume of dentin, the healthy natural ones exhibit increased saturation and are very beautiful to behold.
First I applied the saturated core out of Chroma-dentin and Dentin A3, mixed in even portions, by means of my trusty paint-spatula (Figure 14). It was designed to be thicker and larger, on account of the more solid and saturated natural canine.
Prior to the main aesthetic layering I chose to add some in-depth characterisation to the root-portion around the apical area. Here I applied some undiluted Transparent (died grey for better visibility – Figure 15).
Next I layered and shaped the Dentine A3 core, forming three distinct mamelon sections. *I will later use those to demonstrate ways of modification on how pronounced our dentin core will appear in the incisal third (Figure 16).
As already mentioned, when I need to add illusion of depth, but also increase the definition of our mamelons, Transparent is layered between the dentin horns (Figure 17). In this case I applied some Transparent below the cervical line as well, thinning it down along the central ridge or the root.
A more optically defined mamelon structure
Here I decided to produce a more optically defined mamelon structure in the medial third of the tooth. Also, a more natural, less pronounced, dentine core in the distal part. That is why I covered the entire incisal section with Enamel. In the distal half of the crown this was directly applied over the Dentin A3 mamelons. There was no transparent between them (Figure 18). Some Enamel was also applied irregularly on the root of our anatomical representation, in places overlapping and covering the before layered Transparent.
After resting face down for 10-15min. on a dry paper towel, the porcelain build-up was freed from the wet handkerchief still holding its dorsal side. Naturally all the defects and unassembled areas of porcelain were corrected and smoothened prior to firing (Figure 19). Here you can clearly see the difference in structure around the “dentin horns”. In the medial half of the crown the defining Transparent layer becomes visible (white). Firing was commenced on heat-resistant wool, as described above.
The lingual side
On the next image you can see the vestibular half of the canine immediately after it had cooled down (Figure 20). Even though the lingual side was still to be applied and additional definition added, we could already recognise the increased definition and translucency in the medial half of the incisal section.
For the lingual side my preferred approach was to rebuild most of the volume with the same intensified mixture of Dentin A3+Chroma (1:1). However, without covering the incisal third of the lingual side yet (Figure 21).
At this point it was possible to further influence the aesthetic appearance and visibility of our mamelons. Simply, by applying more or less translucent materials to finish the lingual side and thus provide the said horns with a stabile shade support or, respectively, more transmitted light coming from behind.
Here I layered the medial half of the lingual surface with Transparent (died dark blue) and the distal half with Dentin A3 (died red) (Figure 22). This will further increase the difference in appearance between the two halves.
Japanese master-ceramists
This is a universal tool in modifying the appearance of the icisal portion of the restoration. The lingual layering. When you are limited in space and need to hide the coping and stop it from shining through, after you’ve completely finished layering the vestibular surface cover the lingual side of it entirely with Dentin (where you can even use lighter shades of Dentin, which would reflect more light, or simply use opacious dentins, or shaded dentins to add warmth from within).
You needn’t wary – the vestibular aesthetic cover is composed of well shaded and saturated materials. They will produce the desired appearance because of the properties embedded in them. The lingual layering modifies only the quantity of light that passes from the lingual “direction”.
It is therefore possible to make the mamelons even more visible. By layering the lingual surface of the incisal third with a transparent material. Be careful, however, not to extend this material too far down and reach the incisal margin of the opaque-covered coping. Then even in sufficient thickness of facing material you can make the outlining of your metal support visible.
Of course darker materials tend to absorb more light. When combining darker shades with increased translucency you would add even more visual depth to the situation. For example some Japanese master-ceramists layer their demo-teeth with grayish transparent from the lingual side. This makes the mamelons structures more pronounced and appealing to the spectator.
Next I finished the lingual shape of the canine with undiluted Enamel (Figure 23), with some Dentin A3 (dyed red) layered over the pure Transparent I used for the medial half. Again, according to the sandwich technique for reverse layering.
Finishing application on the vestibular surface
While the lingual build-up was drying I performed a finishing application on the vestibular surface of the crown, with a discrete crack imitation in the middle, to further differentiate the medial and the distal halves (Figure 24). Again a mixture of Enamel and Transparent (1:1) was used to cover the entire vestibular surface of the crowns.
As with all previous work – properly optimised firing was carried out.
Both teeth were mechanically processed with abrading tools and polished with rubber wheels prior to glazing. Glaze material was mixed with the corresponding liquid. It was applied only to the crown-sections of the canine and central incisor. The roots were left unaffected.
After firing the glazed surfaces are selectively polished with fine-gloss silicon wheels and felt discs, in combination with polishing paste for porcelain. This gives our teeth a very natural look. It avoids any chance of artificial glass-like surfaces entering the mouth of our patients. IF we leave the ceramic crowns too smooth the saliva will not be able to wet them. This is because of the very high surface density, exhibited by the glossy porcelain.
Our patient will then have his/hers tongue constantly probing the restoration since it will always stay dry to the touch (unwetted by the saliva) and will thus feel as a foreign object, which would defeat the purpose – integration in the natural dentition. Not to mention that without proper polishing even the most expensive ceramic will look like glass. While with the proper mechanical treatment before and after glazing/self-glazing the most basic material can look like a masterpiece.
Aproximal enamel ridges
The readers must wonder why I am paying so much attention to the aproximal enamel ridges on the vestibular surface. We remember that the masters have taught us to use darker enamels for the aproximal ends of the incisal third. This creates illusion of depth and the all important “wrap-around effect of the natural enamel”. That might still be the case with some single crowns, where more than enough space is available for ceramic layering.
This advice does nothing to help us in situations where bridge work is involved. In those scenarios optical, as well as mechanical, separation is needed. Particularly if we are to make the units look even remotely like separate teeth.
We were then taught that if we apply warm colors between the units it will all be fine. Well – it isn’t, and when inexperienced ceramists are involved it gets even worse. When they apply the “warm” shaded masses interdentally prior to second firing, they usually end up with a “white picked fence” of porcelain blocks that look nothing like teeth, separated by orange or brown lines.
In order to make our teeth really stand out like separate units, we need the healthy appearance of the aproximal enamel edges. They define the aesthetic appearance of the vestibular surfaces. When you define them properly, and enclose underneath a more translucent material, the interdental spaces will look far deeper. The units will appear almost separate from each other, while still remaining part of the same rigid structure.
Porcelain layers
Just like the onion, more layers means more levels of entrapped light. Thus, more porcelain layers, even if consisting only of three to four basic materials. This will make our kaleidoscope as vivid and beautiful as mother nature’s. Some colleagues pretend to be achieving higher aesthetic results by only using mixtures of materials. They almost never apply the pure form of the intended masses.
Well that to my opinion is too subjective, time-consuming and at all unsuitable for a lazy man like me. Still to the unprejudiced viewer such a “masterful” approach might look close to magic. In most cases it follows a very boring and trivial pattern. With the mixtures being applied in single layers, no significant improvement is achieved.
When the different materials are mixed there is no interaction between the varying optical mediums inside the crown. Hence, no multitude of appearances is achieved. Such restorations tend to look well only during demonstrations, but manifest a two-dimensional look in time.
Creating different effects (Figures 25 to 28)
On the next few images you can recognise the different effects I achieved with three basic materials. The Chroma-material would be used in everyday work only for increasing the saturation of cervical, aproximal and interdental areas. You can clearly see the more pronounced dentin core in the medial half of the canine. There is a more natural and smoother transition between dentin and enamel to the distal. You can easily notice the mamelons, crack imitations, incisal halo and varying appearances under different light and view angles.
All of the above demonstrated techniques are universally applicable with every modern ceramic on the market. This is because they have been based on the laws of physics and optics. Rather than on subjective material colour behaviour. That can severely vary from one porcelain manufacturer to another. Keep in mind that minor adjustments might be necessary.
It depends on the basic material on which a certain system is based. For example, feldspathic ceramic systems tend to be more saturated and ever so slightly less transparent than synthetic ones.
The semi-synthetic products are even more differentiated. Every manufacturer seems to be developing those in an individual direction, rather differently from the rest. So, it’s up to the technician to learn the optical properties of the ceramic at hand well. Nevertheless, those individual adjustments are very easy.
Even an inexperienced ceramist will be able to modify the sandwich technique of reverse layering to fir their needs perfectly. Of course feel free to experiment with the materials as you see fit, I actually count on it. Just please keep the laws of physics in mind.
Read part one of this article here.
This article first appeared in Laboratory magazine. You can read the latest issue here.
