| Skip this if you aren't interested in
glazing, or having someone else glaze, a bisque piece. However, if you are going to try
your hand at glazing or if you are interested in buying pieces for other people to glaze,
here are some clarifications and cautions.
There are many types of ceramic pieces out there. The main three are porcelain, bone china and earthenware. Each type of clay has different properties, such as porosity and shrinkage rate. Not all bisques can be treated the same! If you buy a porcelain, it may already have a glaze but you don't know it because the seller wasn't aware of it. Many of the European "bisques" are really glazed white pieces. Since they already have a glaze on them, this limits the kinds of paints and the firing temps that the piece can be done with. Even plain white bisque porcelain may have a shrinkage rate different than the glaze that is usually used by "hobby" people, and this causes crackling, crazing (technically, "shivering") and may eventually cause the new paint/glaze to pop off in chunks. Then the piece is ruined. For good. Bone china is usually compatible with both underglazes and overglazes. Earthenware can be also compatible with both methods. Porcelain, whether bisque or white glazed, is a much more iffy proposition, and we really don't know very well how each one will react. Very dense porcelain shrinks at a different rate than our regular glazes, and the result can be crazing and surface tension. Unless someone is very technically knowledgeable about glazes and clay bodies, it is risky to try glazing pieces whose shrinkage rate is not understood. As far as firing off the acrylic paints that have been used to cold paint a piece, it is not as simple as you think. I have fired about a dozen pieces, variously painted with gessos, acrylics, primers, etc... and NOT ONE has fired clean enough and white enough to glaze. If someone has had better success I would love to hear about it! What I have gotten is a discolored, friable surface that is not safe to use as a basis for any subsequent fired underglazes or glazes. Some paints fire off better than others, but none of them are entirely gone no matter what they are, at least in my experience. So, be very careful when buying bisques, especially unusual ones, and ask someone who has experience in glazing them. Be aware that something may go wrong and be willing to risk your investment, or don't buy it! Good luck! Joanie from Pour Horse |
