This is an example of what happens in a gas kiln when you get both an oxidizing and reducing flame aimed at the same object The base skirt of this two part cemented 1902 Fred Locke insulator demonstrates one of the most graphic examples of this I have seen, the glaze when first applied was of uniform composition . It is an iron based slip glaze. The early industrial coal fired bee hive kilns these were fired in had very poor atmosphere distribution causing effects like this on some pieces . Wile this was not welcome by the industrial users of these insulators we collectors treasure pieces with interesting colors and character like this and value them at a premium.