In his online monograph (Iron, Steel and Swords [https://www.tf.uni-kiel.de/matwis/amat/iss/index.html]
), Dr Helmut Föll (materials scientist) says that crucible damascus steel swords contain no slag because the steel became molten inside the crucible (see https://www.tf.uni-kiel.de
› matwis › amat › iss › kap_a › backbone). The high carbon content of the steel was what allowed it to melt inside the furnace. If I understand it correctly, the heavier weight of the steel caused it to collect on the bottom of the crucible and the lighter slag rose to the top. I am just mentioning the above. I don't know enough about the matter to say whether Föll is right or not. From what I have read, wootz investigator Dr. Zaqro Nonikashvili has used a variant of this principle to charge his crucibles. He divides the iron into two equal portions. One portion is put into the crucible and is covered with ground glass. The second portion is mixed with carbonaceous material and is placed above the the ground glass. The glass becomes molten long before the iron becomes hot enough to absorb any carbon from the carbonaceous material. As the iron above the molten glass heats up enough and begins to absorb carbon, the molten glass forms a barrier that prevents the iron in the bottom of the crucible from absorbing any carbon. The carbon rich alloy in the top of the crucible is heavy enough drop down through the glass but the excess carbon is not. The carbon rich alloy then gives up some of its carbon to the iron in the bottom of the crucible. That iron becomes high carbon steel and melts.