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!)

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