Soluble salts are metallic oxides that are soluble in water such as copper chloride, cobalt chloride and others. I began using soluble salts in foil sagger firing in gas field Raku kilns. The aluminum foil decomposes around 1400F I started firing in ceramic saggers to hotter . Thought the need to use saggers was necessary to protect the interior of the kiln. After many firings I have tried firing them unconfined inside the kiln without any noticeable contamination on the fiber. I would not fire them in electric kilns because of potential corrosion to the elements.