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Waterproofing and Sealing Ceramics
I often get asked this question. First, if you are firing stoneware or porcelain, read Tip #17 about vitrification of clay. Your best means of achieving a waterproof piece is to fire your clay body to the right temperature. But there are cases where you cannot make your ware completely waterproof.
For example, Raku firing does not achieve high enough temperatures to make the clay waterproof. Low fire clay also is not waterproof. Both will leak over time, if you leave water in them. This will probably happen even if you use glaze, because it is very difficult to get a perfect clay/glaze fit which will expand and contract together and not craze. (I have a mug that is a cone 10 stoneware fired at cone 6; it is fine with cold water, but when I put hot water in it the bottom becomes damp. What is happening is that when heated, the glaze probably expands and exposes tiny cracks, the hot water seeps through, and then through the clay. This will weaken the clay over time, and this piece definitely should not go through the dishwasher or in the microwave!)
The most common things I have heard potters say they use for waterproofing are:
Tung Oil (available in hardware or woodworking stores
Thompson's Water Seal (available in hardware stores, used for sealing wood and concrete)
Acrylic Floor Polish (ie Futura.) (available in the grocery store.)
Note that you should test any of these solutions because their waterproofing ability varies. And they will not work on air-dry/oven baked clays to make them durable and waterproof. Remember that kiln fired clay has changed into a glass-like form, and air-dry/oven clay has not. Also, you cannot use any of these for dinnerware. Just for vases, flowerpots, etc. And some people use them over glazes just to get a nice subtle shine, or on the bottom of pots to make them less likely to scratch furniture.
Another reason potters want sealers is to protect painted work. For example, people that use acrylic paints on bisque and want to protect the paint. We do sell Duncan spray and brush-on sealers in flat and gloss, and Amaco All-Purpose Sealer which will protect the paint from chipping and smudging, and give it a shine if you wish. They are good for decorative pieces. But these are not waterproof. So if you want your piece to be waterproof, try one of the above products. But since those items work by penetrating the clay, and they probably won’t penetrate through the acrylic paint, I would put it on the inside of the piece, then protect the painted outside with an acrylic spray sealer like the Duncan products (or similar items available in the hardware store.) So for example, if you want to waterproof a flowerpot, put the tung oil, Thompson’s water seal, or acrylic floor polish on the inside of the pot. Paint and seal the outside.