Pottery and Ceramics Glaze
Glazing Defects And Remedies
There are many things that can go wrong with a glaze.
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One of the most common glaze defects is crazing, although in some cases crazing is a sought after effect, in which case it's called a Crackle Glaze.
Crazing will occur when the glaze doesn't fit the clay body.
The glaze shrinks more than the clay during the cooling causing the glaze to crack like a spider web.
Crazing can occur after a piece has been fired.
You can hear a bowl or platter making a pinging noise long after firing, this is the glaze cracking.
Crazing can also occur over centuries, as can be seen in many older wares, when they were new, they would have looked OK.
Add silica to the clay body
Add silica to the glaze
Add alumina to the glaze
Soak the kiln
When small dots of unglazed or depressed areas appear in the glaze surface, this is called pin holing.
Pin holing happens when gases in the glaze and clay bubble up to the surface.
The gas bubbles pop and a hole appears, which doesn't heal over.
Fire the glaze higher
Soak the kiln
Wipe all dust off of your bisque
Spray bisque lightly with water prior to glazing
When a glaze has a bubbled, lava like surface, it is called blistering.
There are some so called lava glazes which are specifically formulated to blister.
This effect occurs when the gases that are released from the glaze become trapped in the glaze form before they can escape and the glaze smooth over.
This can be due to underfiring or overfiring.
The glaze pulls together and forms separate droplets on the surface of the clay body. Crawling occurs when the glaze contracts too much during the heating cycle and doesn't get a sufficient chance to smooth over.
Another cause may be due to bad adhesion of the glaze, for instance when glaze is applied to a very dusty pot or the bisque is fired to hot resulting in it not being able to absorb the glaze when it is painted on.
Remedies:Substitute some of the clay content of the glaze with kaolin
Substitute glaze fluxes such as zinc oxide and dolomite in their calcined form
Clean off any dust with a sponge prior to glazing.
Shivering happens when a glaze doesn't shrink as much as the clay body.
When this happens parts of the glaze crack and move against each other lifting off from the clay surface.
This can be so bad that pieces of glaze will actually fall off the surface.
Remedies:Reduce silica in the clay body
Reduce silica in the glaze
Reduce alumina in the glaze
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