When you wash dark clothes for the first time - use cold water and put salt in the water. The salt helps set the color. When I want to brighten dark clothes after they have been washed several times, I repeat the procedure (salt in cold water when washing).
This tip is from 2001 so I am not sure if Lou is still with us or not. I did however find this tip on an extension site recently and the recommendation was 1/2 cup of salt.
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Use 2 Tbsp. turpentine and 1 small handful of salt. Pour boiling water over this mixture. Put garment or cloth in and saturate. Let it soak until the water gets cold.
