So you're mixing your own glazes (or god forbid, engobes) and now you have a bucket with powder and water. Does it drip down the side of pots? Does it apply too thin or thick? Does it crack or flake off when dry? If you said yes to any of these, you can fix these by modifying the rheology of the slurry.
The issue is that different issues with the glaze can cause the same symptoms, so fixing the slurry is highly dependent on your specific situation. If you apply the wrong solution to thin or thicken the glaze, you may end up with the same problems.
The ideal glaze goes on bisque in an even layer without drips, at the desired thickness, hardens within 10-20 seconds, and completely dries relatively quickly. When dry, the glaze does not crack or flake off.
First, make sure your glaze has the right amount of clay. Not too little, not to much. Glazes should have 10% to 20% kaolin or ball clay. In the absence of either, 1% to 2% bentonite will suffice.
The amount of water added to the dry powder should be a part of the recipe itself. This is because the clay content in the recipe and the desired thickness. If the recipe does not list the water content, you need to test yourself.
Assuming the clay content meets the guidelines above, a good place to start is 80g of water for every 100g of powder. Then slowly add water until the glaze is thin enough. This requires glazing and firing and seeing the results. But once you do find a good amount of water, you can record the specific gravity of the glaze.
To find the specific gravity (s.g.), measure the density by weighing 100ml of glaze then dividing by 100. Liner glazes, which like to go on thin, can have an s.g. as low as 1.3. Most glazes fall between 1.3 and 1.5. Shino Glazes are absurdly thick and can have an s.g. over 1.6. For reference, pure water has an s.g. of 1 by definition. Feldspar, a major glaze ingredient, has an s.g. of 2-3. In other words, a glaze high in water is closer to 1 and low in water is closer to 2.
Even if your glaze has the right clay content, the water content that gives the best thickness may not gel well. A well flocculated glaze clings to the ware and goes on in an even layer. But note that many glazes may have the right viscosity with just water. The most common need for flocculants comes when you want the glaze to go on thinner, but adding water removes the glaze's ability to gel. In this case, add some epsom salts.
If a glaze is too watery, add Epsom Salts in small quantities (<1g per 100g of glaze). Slowly add more if the glaze doesn't thicken. However, note that if there is too much water, some glazes may never thicken up. The glaze should become viscous - after stirring the glaze, the vortex continues to spin for a few seconds but then comes to an abrupt halt. This is what you want. If it stops spinning instantly, it may be over flocculated. If it continues to spin (like water), it is under flocculated.
If the glaze becomes over flocculated for its water content, don't add more water. A few drops of a deflocculant like Sodium Silicate will thin it back out. Deflocculants are strong too, so add a few drops per 100g of glaze at a time. It's best not to go back and forth too much because then both flocculants and deflocculants will lose their potency.
In fact, a glaze with no clay at all can be made viscous and dippable! Epsom salts can suspend a mixture of pure feldspar for example. However, the dried glaze will fall off the bisque if you just look at it (it needs clay to harden).
A well flocculated glaze can even be applied to non-porous surfaces. This is especially useful for reglazing pots. Tony Hansen used epsom salts to glaze a spoon.
Engobes are like dipping glazes, but are used to apply a thin layer of clay to leather hard pots.
Since engobes are naturally high in clay, but that can be a good thing. Unlike glazes, it can be desirable to have an engobe that is 70% or more clay because then it will shrink with the pot as it dries.
The water content of the engobe should also be minimized. If there is too much water, the pot will become saturated after application and will crack or collapse. The water content is similar to glazes, about 80 to 100g of water for every 100g of powder.
Therefore making an engobe apply in a thin even layer is left completely to the use of flocculants and deflocculants. After mixing the powder and water, the engobe will be a thick slip. Add a few drops of deflocculant and it will quickly liquify. Add as little deflocculant as possible until the engobe turns runny. Then, slowly add flocculant until it reaches the desired viscosity. You may need to use several test pieces to find the optimal amount.
In my experience, engobes get thicker in storage, especially newly mixed ones. The rheology needs to be tuned every time before use.
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