Article: A preserving pan is a good size for this, to allow room for the fabric. Dip it into the pan and wet it completely. Put into a non-drip plastic basket to take outside. Hang on a line, over the basket, or over a clothes rack. Expect some stiffness compared to the original fabric but it should mold to the shape needed with bending.
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In a large pan, mix 1 lb of alum with 1 pint hot water from a faucet. Select the fabric that you wish to treat. Pull out the completely wet fabric. Use once dried.