Thixotropy and How to Gel a Ceramic Glaze

I will show you why thixotropy is so important. Glazes that you have never been able to suspend or apply evenly will work beautifully.

B. Glazes

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Please excuse the poor framing of this video, I did it in the early days of eyeglass camera tech (using Pivothead glasses).
If you mix your own glazes for bisque ware, understanding the difference between specific gravity, viscosity and thixotropy will change your life as a potter! Watch this video to see an example of a process, but pay more attention to this text to get a broader picture.

When mixing a glaze for the first time it is necessary to discover the right specific gravity (this depends on a number of factors). Once established then optionally gel the slurry to improve its thixotropy.

How does one know what specific gravity to target? Start with a 100:80 ratio of well-mixed glaze powder to water. For almost all glazes this produces a slurry that is too viscous. Add water, while propeller mixing, until stopping the mixer leaves the slurry in motion for 2-3 seconds before it stops. Measure the specific gravity and do two simple tests:

1. Put your finger, or some other stirring implement, into the slurry and extract it. If the glaze hangs on, covering it evenly with only a couple of drips falling off, then it is ready.

2. Sedimentation testing: Let it sit for a few days. If no layer has settled on the bottom it is likely OK to try using as-is.

If test 1 or 2 fails then "thixotropy improvement" is now needed. Add water until the glaze stays in motion for 10 seconds after shutting off the mixer. Now, add the flocculant in small amounts, while mixing, until the slurry begins to gel (when right it should again stop and bounce back slightly after 2-3 seconds). Be careful not to add too much flocculant or it will turn into jelly (mix it with an unflocculated fresh batch to retrieve it). Let it sit again for a day or two. If it has settled then repeat the process of adding water and then flocculant (being careful not to add too much water, there is a point where it will no longer gel). When it is right, measure the specific gravity and remember that as the starting point for future batches.

There are special cases to consider:

1. The glaze has almost no clay. Add 2% bentonite, the extra particle surface area it imparts makes the flocculant much more effective.

2. The glaze contains significant clay (25% or more). The clay should suspend and gel the slurry enough to make it naturally thixotropic (no further action needed). Of course, even higher percentages of clay will cause other issues (e.g. excessive shrinkage on drying).

3. The glaze contains significant Gerstley Borate (GB). GB has the suspending power of ball clay. If there is 15% or more the slurry should gel naturally. If there is 25% or more it will gel too much and need a deflocculant.

4. The glaze contains significant GB and also clay. This is a problem, these require too much water and gel too much. Consider fixing the recipe by learning how to source B₂O₃ from a frit rather than GB (there are videos and articles here on doing that).

Links

Glossary	Thixotropy Thixotropy is a property of ceramic slurries of high water content. Thixotropic suspensions flow when moving but gel after sitting (for a few moments more depending on application). This phenomenon is helpful in getting even, drip-free glaze coverage.
Glossary	Specific gravity In ceramics, the specific gravity of slurries tells us their water-to-solids ratio. That ratio is a key indicator of performance and enabler of consistency.
Glossary	Viscosity In ceramic slurries (especially casting slips, but also glazes) the degree of fluidity of the suspension is important to its performance.
Materials	Gerstley Borate Gerstley Borate was a natural source of boron for ceramic glazes. It was plastic and melted clear at 1750F. Now we need to replace it. How?
Recipes	G2934 - Matte Glaze Base for Cone 6 A base MgO matte glaze recipe fires to a hard utilitarian surface and has very good working properties. Blend in the glossy if it is too matte.

By Tony Hansen
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