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Bare Spots in Slumping Molds


Slumping molds made from ceramic (or bisque, as potters would say) are generally easy to kiln wash, but on occasion these molds contain a small area that remains white and resists being covered by kiln wash.  These bare spots tend to remain even after thoroughly cleaning the mold and then reapplying kiln wash.


The bare spots, which are known as "hard spots" or "hot spots" by potters, are caused by the mold-making process.  When the liquid clay suspension is poured into the plaster mold used to produce the slumping mold, the force of the clay hitting the surface of the mold causes a change in the orientation of the molecules of the clay.  This change results in the creation of a hard spot in the clay.  The spot resists water based liquids (such as kiln wash), and results in an area of the mold with different water absorption characteristics.


The spot can be minimized or eliminated by sanding with a Scotchbrite or similar pad.  Alternatively, full coverage can be achieved by applying a thick coat of kiln wash over the area, allowing it to dry completely, then brushing lightly over the area when applying further coats. 


Even easier, in most instances it's ok to just ignore the bare white spots and use the kiln washed mold anyway.  So long as your slumping takes place below 1300F/700C (the lower the better), the bare spots will not stick to your glass, and the mold will work just as well as if it were perfect.


Copyright 2006 Brad Walker.  All rights reserved.

Special thanks to Tom in Texas for his assistance with this writeup.

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Copyright 2005-2006 by M. Bradley Walker.  All rights reserved.

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