Judi Buchanan’s — OOPS !!! CORRECTING GLAZE FLAWS

OOPS !!! CORRECTING GLAZE FLAWS

 

The most common glaze flaw is BARE SPOTS on the pot usually caused by the glaze cracking away from the pot as it dries. This is called crawling,caused by applying the glaze too thickly especially if it has a lot of clay in the recipe. If you see cracks in the glaze on an unfired pot you can some times rub it with your finger tip and blend it back together but if the glaze is loose you just have to scrape it off and retouch that area. Some glazes are so high in clay ,which shrinks as it dries and as it fires, that they are only suitable for using on greenware or very thinly on bisque. Dust on the bisque will keep the glaze from sticking to the pot also leaving bare spots these can usually be seen before firing if you check. Wipe down bisque with a damp spongue or rinse to prevent this flaw. Grease or wax repel the glaze so wash your hands and don’t use lotion before you handle bisque. Many times a wax drip can be scraped then sanded off with a with a carbide dremmel bit. I recommend all studio wax be tinted with food coloring so it is easy to see when you have a drip.

PINHOLING is caused when air in the pores of the body is forced out through the glaze. Moistening the pot to force the air out is one answer. Firing the bisque a cone or two hotter may also help. If the holes are noticed before firing they can be smoothed over by rubbing.

BUBBLES on the surface of the glaze is called CRATERING and is usually caused by over firing. Fired past the melting point or held at the top temp too long causes the smooth melted coating to begin to bubble. A thinner glaze coat fired a bit lower or soaked about 200 degrees below top temp should stop the problem. A pot may be corrected by sanding off the rough bubbles, applying another thin glaze coat and refiring. A glaze is corrected by adding clay and silica.

CRACKLE is considered a flaw on functional pottery. If the body isn’t vitrified (fired until it doesn’t absorb water) the pot may not hold water and it is weakened by the a wetting and drying. Bacteria also might grow in the cracks . This is caused by the glaze contracting more than the body as it cools.The answer to this problem is to correct the glaze recipe by adding chemicals that contract much less. Talc is a combination of magnesium and silica and adding small amounts of this will usually correct without changing the charactor of the glaze. Titanium dioxide is also a solution but the glaze may not look the same when this cure is used.

SHIVERING is the opposite problem. When the the glaze doesn’t shrink as much as the body it flakes off as it cools and that is harder to solve with out completely reformulating the glaze with fluxes that are high expansion like sodium and potash. DUNTING is this problem in the extreme. The pot is broken, shattered or spiral cracked.

Judi Buchanan,Flutter-by Pottery

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