text: Although all bun ingots share the same basic morphology, the details of their form and the texture of their convex base is dependent on the mould in which they cooled. Bun ingots made in purpose-dug depressions in sand can be highly variable in form even on the same site, whereas ingots cast in reusable moulds will form sets of identical “mould siblings”.
  The composition of the metal and its cooling conditions affect structure. As the ingot cools gases are released giving the upper surface a “blistered” texture and if cooling takes place outside of the furnace, the outer surface often becomes oxidised. Casting in a warm mould or reheating furnace gives the ingot an even columnar structure running in the direction of cooling, whereas ingots cast in a cold mould have a distinctive two stage cooling structure with an outer chilled layer reflecting the rapid cooling of the bottom when it came into contact with the mould. A slightly concave upper surface can be produced if the top of the ingot cools more slowly than the bottom.