Our waste glaze always fires a frothy volcanic black which breaks on handling.  We likely have too many raw materials going into the slop tank.

My scrap glaze comes from other complete glazes. Rinsing off dippers, buckets, brushes, washing out the inside of my spray booth, wiping pot feet, etc. No clay or isolated raw materials should be be included that would throw the mix out of the limits range of a normal cone 6 glaze in terms of the ratios of flux to refactory to glass formers. Below is my first scrap glaze (the gray green transparent). My second batch is less green, but otherwise similar.