| Answers from the CM Technical Staff
Q In making a slip that contains borax, I have had the problem of it
crystallizing once the slip is mixed up. How do I get it to completely
dissolve so that it can be sprayed through a spray gun without clogging
the tip? Are there certain types of borax that are finer that will not
clump once mixed up?—J.D.
Borax is one of only two or three clay and glaze materials that we
commonly use in sizable quantities that is water soluble. Each soluble
material has a solubility that varies in a different way with water
temperature.
Borax is very soluble in water, usually the most soluble material in a
formulation. You’ve found one of the problems that can result from
borax solubility; crystallization. This is caused by the sensitivity of
the solubility of borax to water temperature. Simply put, borax is many
times more soluble in hot water than in cold water. A common result of
this characteristic is that some of the borax that dissolves when the
slip is originally mixed will precipitate later. This can happen if you
mix your slips with hot water, if your studio temperature fluctuates,
or if you leave the lid off for enough time for the liquid to cool due
to evaporation.
Initially dissolved borax will typically crystallize into large
crystals, usually at the surface of the liquid. Among other problems,
these large crystals can clog sprayers, as you describe, and can make
for uneven material distribution in dipped glazes.
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Borax crystals are difficult to avoid, but they aren’t very hard to
deal with, either. You can approach the problem in two ways: 1) avoid
excess dissolution; 2) completely dissolve the borax.
To avoid excess dissolution, use cold water to mix your slip, avoid
temperature fluctuations during storage, don’t leave the lid off any
longer than is necessary, and mix smaller batches. For this approach to
work, you would first want finer mesh powder borax that will flow
through sieves and sprayers. Many ceramic suppliers stock this powder
borax in addition to the more common granular borax. You will want to
store borax in an airtight container.
Alternatively, to completely dissolve the borax, simply heat the
quantity of slip or glaze you need for spraying, either in a microwave
or on a hotplate. With a little stirring, the large crystals should
quickly disappear.
Keep in mind you are likely to see differences in your results from
these two different approaches, as they will result in different
concentrations of borax in the slip.
Sometimes the variability caused by soluble materials is the reason we
use them. However, we usually want to avoid the variability caused by
solubility, because it is difficult to control. If your reason for
using borax is as a source of boron for your fired slip or glaze, then
there are several stable frits (synthetic glass powders) that are made
with boron that can often be used to replace borax. With the aid of
glaze calculation and careful testing, you can avoid troublesome
materials. You’ll also be in a better position to assess the
suitability of a glaze or slip recipe.
David Pier
Chapel Hill, North Carolina
e-mail technical questions to editorial@ceramicsmonthly.org
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