How to achieve the perfect glaze fit in clay work
by Tim Reed

Question:
Generally speaking, which of the following conditions succeed in not only achieving an optimal glaze fit, but also serves to increase the overall strength and durability of any clay work?
 
A)  Perfectly balancing clay and glaze shrinkage rates so that no tension exists       between them.
 
B)  Including more Independent Party members in the White House.
 
C)  Establishing a certain, controlled amount of tension between the clay and
      its glaze.
 
D)  Presenting the clay work with an offer it can't refuse (... essentially, intimidating
      and threatening violence if it doesn't comply).

Answer:
Well, to begin with, answers "B" and "D" may very well have a certain bearing on matters of social policy, but studies have indicated that the average clay work is unimpressed and remains relatively unaffected by such influences. Also, even though 4 out of 5 clay works surveyed prefer the peaceful, less stressful lifestyle, a total lack of tension (as in answer "A") has little effect in regards to strengthening or increasing durability in the overall piece.

However, if you chose "C" as your answer, then you are exactly correct, and here's why...

You see, totally eliminating tension of any kind from clay work would seem to be a very reasonable objective, especially since we are dealing with materials that are relatively easy to damage anyway. Yet, the perfect lack of tension in such a vulnerable material would only be practical provided that its immediate environment was constantly protected from hostile outside influence. Problem is, until we all decide to function as though we live and work in a museum, it's highly unlikely that the perfect environment will be found where our prized creations will always remain completely safe from harm.

But there's a clever little trick you can use to actually strengthen the entire piece and reduce the possibility of future damage. Essentially, the idea is to create a slight amount of tension between the clay and its glaze. Such a tension, engineered correctly, will ultimately result in the additional strength that you wish to achieve the key to this trick is found in controlling a particular state known as "compression".

The compression referred to here is achieved by allowing the glaze coating to be ever-so-slightly "larger" than the underlying clay. If this seems a little unclear to you, then let me take a moment to expand on just how that works...

You see, there are three basic conditions possible in which the fit-relationship between the clay and its glaze will eventually find itself: Either the state of being in "compression", "contraction", or that of "neutrality". For our purposes, the "neutrality" mentioned here is simply one in which no tension exists whatsoever between the clay and glaze.

However, many people confuse compression, with contraction, which are two very different issues, indeed. In fact, understanding this difference will ultimately determine your level of glaze-fitting success. Therefore, let's go a little further in considering the differences between these two.

The "contraction" of a glaze simply refers to the amount in which it would naturally shrink when given the chance to freely do so without restriction. The contraction of a glaze is definitely something you will want to monitor very closely. For instance, "crazing" in a glaze occurs when the glaze contracts so much around the underlying clay form that it becomes stretched too tight for comfort, literally splitting itself apart.

On the other hand, "compression" of a glaze will occur when its underlying clay ends up shrinking more than that of the outer coating of glaze itself. In such a case, the glaze eventually becomes forced to react by compressing against itself. Allowing too much contraction, however, will result in the glaze being compelled to shiver away from the clay. In extreme cases, especially when the underlying clay form is weakened by an inherent structural defect, the buildup of compressive pressure can become so great that it could literally crush the clay work.

NOTE: Keep in mind that the contraction that we're talking about here is a condition where the glaze contracts against ITSELF... Not against the clay form that it surrounds. We are NOT looking to create tension with the idea of further compressing the clay form underneath.

So, how do you get this beneficial state of compression to occur?

Well, as eluded to above, the easiest way to do this is to establish a relationship where the overlying clay shrinks slightly MORE than its overlying glaze. As the clay does this it begins to exert a force which tends to pull the glaze in along with it.

Even so, the chemical nature of each particular glaze will allow it to be pulled only so far before it begins to resist. This point of resistance is what you're looking for. As the glaze moves further into resistance it begins to become more dense and compacted within itself. The natural result of this compaction leaves a stronger glaze coating, more resistant to damage from outside influence.

NOTE: Studies indicate that a glaze fitted in this manner will actually increase the strength of clay work by up to four times that of an unglazed piece (yes, 4!). The moral of that story is this: Fit your glazes well to the clay you chose to use, for in doing so you will avoid a multitude of trouble, and your clay work. will likely live a longer, more prosperous life.

TIP: Considering the powerful pressures that glaze can exert on clay work., be sure to equalize these pressures by applying consistent glaze coverage to BOTH sides of the clay wall when possible. This is especially important with clay work. designed for durability such as dinnerware.

The explanation for exactly how to chemically achieve such a condition between the clay and glaze is beyond the intended scope of this particular article. Indeed, each individual clay/glaze combination will easily have its own set of chemical/ingredient choices for doing so. However, much has already been written and made available for free on this subject, and finding the information pertaining to your particular situation will probably not be difficult at all.

For those who are looking for more technical info on how to adjust your clay or glaze formulas I suggest looking into the following online sources:

CeramicMaterials.Info (brought to you by Digitalfire Corp)
http://ceramic-materials.com/cermat/index.php
 
NOTE: If you want to really get into understanding and producing your own glaze recipes, then this is definitely a site you want to bookmark! It has one of the best ceramic materials information library around with in-depth articles on what individual ingredients do, not to mention how to use them.

Clayart forum (The link below will take you straight to their official search page)
http://www.potters.org/search.htm
 
NOTE: The Clayart forum is perhaps the first place I've turned to for a long time where you can get just about any ceramic questioned answered. The link above will take you straight to their official archive search page, but you can always get really good specific results simply by typing pertinent key words into a quick "Google" or "Yahoo" search (doing so will almost always take you straight into a discussion already in progress somewhere in the Clayart forum.

The BigCeramicStore.com (a great collection of ceramic articles and tips)
http://www.bigceramicstore.com/Information/CeramicsInfo.htm
 
NOTE: You'll find great tips at this site on how to do all kinds of clay-oriented stuff. The info project has been primarily put together by Cindi Anderson who I think has done a wonderful job there!

These are just three of the great open information sites out there, but they will certainly help to get you pretty far down the road. Indeed, there's plenty of good advice to be found all over the internet on ceramic processes. In fact, I think you'll get way down the road finding very specific information simply by typing a short question in your browser's search bar.

Hope this helps you understand a little more about how to strengthen your clay work and achieve the perfect glaze fit. Good luck... and know that I'm always wishing for your greatest success!

Tim

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