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How to fire mid range stoneware (cone 6). kiln wash flakes and sometimes mars a piece in glaze firing. Therefore, use a layer of sand (the same kind that is sold in plastic bags at KMart for kids' sandboxes. Spread the layer very carefully on the shelf after it has been positioned, approximately 1/8 of an inch thick. If you suspect that the glaze will run then use a heavier layer. Since sand will not melt until cone 28 there is no problem. The benefits are that the sand will absorb any glaze which may run off the pot and therefore the piece can be removed from the shelf without damage to the shelf or the pot. Occasionally some sand adheres to the vessel this is easily hand ground with a stone or machine ground with my Makita grinder. It is important to remember to put the sand on the shelves after they are in the kiln and keep it 1/2 " away from the edge of the shelf in order to avoid contamination. I have not had any problems with sand particles getting in my glazed pieces and I do have a venting system. The secondary benefit is that pieces with large flat bottoms never warp! I also use the same technique for bisque firing all platters.