If you've seen old brown insulators either on guy-wires or telephone poles, chances are that they were covered with an Albany Slip glaze. Albany Slip was used by potters, too, and many of them loved it. But it's gone. Because Albany Slip is no longer being dug commercially, people have made various equivalents by starting with other clay and making judicious amendments to it. The object is to get something that you can substitute straight across, so you don't have to develop the glaze recipe all over again from scratch with different materials.
Some people think it' s no good to attempt to replicate materials that have gone missing.
It's better, they argue, to use what's available, and do what you can with it. You see, I have a friend who lives on top of millions of tons of clay. This clay is relatively ordinary stuff, as far as one can tell by looking at it. On the left is a wee lump of the stuff, as dug. I broke it off a much larger lump.