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Brushing Glazes


Almost all potters in the past made their own glazes. Low fire hobbyists traditionally bought bottled glazes. But in recent years many hobby potters have switched to bottled glazes also. Given the cost of these products, this phenomenon is interesting. Web sites selling pottery equipment and supplies (and physical stores) often feature glazes very prominently (a reflection of how popular and lucrative these are).

Problems with brushing glazes are:
-Often they do not fit your clay body (craze, shiver, go milky).
-Brushing is a slow process and it is difficult to get the glaze on evenly.
-Brushing glazes cannot effectively be thinned for dipping, they drip and run too much, go on too thin and take too long to dry. Manufacturers sometimes supply dipping versions of their glazes but these often still do not gel and hold and dry in seconds the way home-made ones do.
-They are very expensive, a pint can cost as much as making gallons of your own recipe.
-They often do not fire as expected.
-Stocks run out more frequently, often at very inconvenient times.
-Old dried-out or water-reduced jars are difficult to restore to working condition and it is impossible to sieve them.

Advantages:
-No hassle, no storage of large containers of glaze.
-Best for classroom situations.
-Labels are clear.
-Special surfaces (metallics, reactives, bright colors, layered effects) are easier to obtain.

Most of these glazes are produced by simply starting with a good base transparent recipe (normally highly fritted) targeted at the working temperature and adding stains, opacifiers and variegators. Brushing glazes do not have special recipes, they simply contain lots of gum (it makes them paint on well and dry slowly). Typically they have been mixed to a specific gravity of around 1.6 in a gum-water mix and put into a jar with a fancy label.

Dipping glaze recipes converted to brushing glazes. Easy and inexpensive.

Dipping glaze recipes converted to brushing glazes. Easy and inexpensive.

These are cone 6 Alberta Slip recipes that have been brushed onto the outsides of these mugs (three coats). Recipes are GA6C Rutile Blue on the outside of the left mug, GA6F Alberta Slip Oatmeal on the outside of the center mug, GA6F Oatmeal over G2926B black on the outside of the right mug). One-pint jars were made using 500g of powder, 75g of Laguna CMC gum solution and 280g of water (a good mixer is needed). This produces a specific gravity of 1.6 and a glaze consistency the same as commercial bottled glazes. The presence of the gum made it unnecessary to calcine the Alberta Slip.

Commercial glazes may not work on your clay body

Commercial glazes may not work on your clay body

Left: Plainsman M390. Right M370 porcelain. The bottom two samples are a popular ultra clear commercial bottled glaze that costs about $13/pint. On the porcelain, it is crazing. On the red clay it is saturating with micro-bubbles and going totally cloudy and even a satin surface (it should be like the transparent above it). It is likely very high in boron and melting too early. Whose fault is this? No ones. This glaze is simply not compatible with these two bodies.


By Tony Hansen




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