For some time I discovered crystals in some glazes that I kept in my (enclosed) glaze buckets. These are self-designed glazes of kaolin, quartz, frits, wollastonite, etc. Mixed with water and peptapon.
Some glazes had small crystals (felt a bit like sand), but some glazes developed large amounts of crystals. In some cases as large as a five euro cent (see photo). These crystals made it impossible to glaze (because glaze spray got plugged), so I started to look for a solution.
The cause
The glazes where this happened I had used for a fairly long time (over 10 years) and started when I moved from Amsterdam to Purmerend. The raw materials are still from the same supplier (Ve-Ka, first directly, now via Arts & Craft in The Goorn) so that could probably not be the reason. Although it is of course possible that the composition of the raw material has been changed over the years (or Ve-Ka got his raw materials from another supplier).
So I assumed it was because of the tap water of Purmerend. In addition, the boron oxide in the frit was probably responsible for the crystal formation. Despite the fact that I use the boron oxide in the form of a frit for the glaze recipe , it appeared from the literature that a part of it still solves in the water.
Solutions I tried
1. Other waters
Because I saw the water as a prime suspect, I started searching for other waters. The first thing I tried was to use previously boiled water and later even distilled water, but that did not help.
2. Other boron oxide frit
The other possible culprit was the alkali boron frit in the glaze recipe. Therefore I replaced the alkali boron frit by a calcium borate (another frit, with a different composition, but by adjusting the recipe the glaze remained the same). Unfortunately, after a while it did not work either. Afterwards I read in the literature that calcium borate probably is more water soluble than the alkali boron frit I used, so I probably increased the problem.
Direct approach of the already formed crystals
1. To sieve crystals from glaze
The first thing I did was to sieve the crystals (80 mesh). That worked fine, but the composition of the glaze changed, which changed the character of it. The crystals contained in all probability the melting ingredient boron oxide, the sieving had as a result an increasing melting point (and influenced the color).
2. To grind crystals
By grinding again the crystals I did not change the glaze but that proved easier said than done. By using a mortar it was possible, but it took me too much time. In the mean time I got a ball mill, with which it would be easier to do, but the main problem was that the crystals came back after some time (a few weeks to a few months). So that keeps one going.
3. Heating glaze (up to boiling point)
By boiling the glaze, the crystals were found to dissolve. This is a good solution for using the glaze again. But a few days later, the chrystals were back.
The Permanent Solution
Eventually I found an article by John Britt (“Flambe Magic”). He used the crystals to get some glaze effects, but also hinted at the solution; it might have something to do with the pH of the prepared glaze. After having taken some PH values from glazes with and without crystals, it showed that the glazes with crystals had a (too) high PH value (9 or higher). It was therefore necessary to add something “sour” into the glaze to lower the pH value.
1. (Household) vinegar
The first most obvious thing was the addition of vinegar. After some tests, the crystals were found to dissolve in the vinegar, but it was too imprecise and after some time the vinegar didn’t work any longer.
2. Magnesium sulfate
In the literature, I found the use of Epsom salt (magnesium sulphate) as a small addition to glazes. The chemical name of this is magnesium sulfate. I added about 0.3% in the concerned glazes. This seemed to work, but the crystals finally (perhaps a little later than usual) came back again.
3. Calcium chloride solution
My lesson learned
Try to solve glaze problems as structured as possible and never give up (well, not too fast) …..
Nota Bene
Recently someone pointed out to me that calcium chloride reacts with bone-ash (bone or calcium phosphate). The result is a hard layer on the bottom of the glaze bucket (M. Bailey, glazes Cone 6, p.90).
I have glazes with calcium phosphate (no bone, but the inorganic variant tricalcium phosphate), but because I have no problems with crystal formation with these glazes I never added calcium chloride in these (if it is not broke, do not fix it) If you, however, have problems with crystal formation in this type of glazes, try to solve it with vinegar or magnesium sulfate.
One more time I’m going to do a test with tricalcium phosphate and calcium chloride, to find out whether and how such a hard layer arises; prevention is better than solving problems!
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