Years ago I read that Warren Mackenzie would scrape the floor of his glaze room and save all that stuff up. He would add some percentage of cobalt and use that as his blue glaze. I think he called it the 'floor glaze' or maybe the 'shop glaze'.
So of course I had to start doing that. I have about one and a half quart jars of floor glaze saved up. In my case it's probably more of a slip. I am planning on testing this in my next firing. I'll have to mix it up, add water, sieve it really well to get all the course dirt and cat and dog hair out and give it a go.