Fine tune the thixotropy of a glaze or engobe slurry

How I tune the thixotropy of a dipping glaze or engobe so it hangs on after extraction, not dripping.

Transcript/Notes

Gelled (thixotropic) slurries for dipping are so much better to work with, you'll never go back once you have mastered this DIY technique. While some glazes and engobes gel naturally, especially those with high clay content, these almost always work best when the water content is within a certain range, so fine-tuning like this is still needed. Although not shown here, if over-gelling happens, a drip or two of deflocculant (e.g. Darvan) brings back the fluidity; this is more likely to happen with engobes since they need more gel (for dipping and even more for painting).

Transcript:
I am going to check the thixotropy of this slurry.
To do that, I turn off the mixer and count until it gels and stops.

more..

In this case, that happens in about 3 seconds.
Then I slowly pull the spatula out.
Notice there’s not one drip coming off of it.
Now, I’ll do it again. A few drips.
I like it to gel just a little more so I’ll add a tiny pinch of Epsom salts and let it mix for a few minutes.
This is the procedure each time I use this.

Note: Since I have a good table top mixer I use granular Epsom salts, sprinkling it in and leaving the slurry to mix until it stabilizes. Fine tuning slurries like this is a lot of about judgement, and I feel more comfortable dealing with the granular material directly.

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.

See the magic of thixotropy as I mix a 20kg batch of G2926B glaze

In this video, I mix 20kg of G2926B glaze powder into 20kg of water using our powerful propeller mixer. The resulting slurry is like water, absolutely unusable. Yet on measuring the specific gravity (using a hydrometer because it floats freely) I find that it is too high, I actually have to add more water! How is that even possible? Instead, I add Epsom salts and mix again and the slurry gels and hangs on in a perfectly even layer when I dip the spatula. This is a thixotropic gel, it will apply evenly to bisque ware yet not go on too thickly. We normally recommend a specific gravity of 1.44 for this glaze, but in this case, it seemed watery enough at 1.46 (on use, it will become clear if 1.46 is OK e.g. if it goes onto the ware too thick). If that happens I'll just add water to 1.44 (and more Epson salts if needed). Based on online pricing at this time, coverage is minimum six times and as much as twenty times less expensive than buying jars of transparent brushing glaze (considering both the total powder weight and the specific gravity difference between this and commercial glazes we use).

Fine-tuning the thixotropy of a glaze or engobe

For dipping, this is so much better!

This picture has its own page with more detail, click here to see it.

Watch this 30-second video to see. Gelled (thixotropic) slurries for dipping are so much better to work with; you'll never go back once you have mastered this DIY technique. While some glazes and engobes gel naturally, especially those with high clay content, these almost always work best when the water content is within a certain range, so fine-tuning like this is still needed. Although not shown here, if over-gelling happens, a drip or two of deflocculant (e.g. Darvan) brings back the fluidity, this is more likely to happen with engobes since they need more gel (for dipping and even more for painting). A side benefit of this: No settling in the bucket.

By Tony Hansen
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