I always keep what is left from clay processing. Sometimes for fun, in that case I use small stones or sand as decoration in my studio, or I mix it to my commercial clay. I have particles separated by 4 different sieves so that way I can increase the diversity these leftovers produce. So with 4 different sieves I can get 4 different clays with variable additives of leftovers. The only thing I try to avoid always are limestone, whiting and dolomite. These are the three rocks very typical for Slovenian geology. They break into dust in higher temperatures or even worse, expand back with moisture from the air after firing, and that expanding can break thin walls of the clay body. If they are small, body can hold expanding, if not, it beaks. Strength of clay body varies from clay to clay and depends of materials and particle size. I find very plastic clay to contain smallest of  particles of clean clay minerals.

I always test it first. Firing it tells me whether unwanted rocks are present or not.

Here is white stoneware clay Georg and Schneider 254 and leftovers of Straža clay (medium size particles):

Here I mixed Rožnik clay leftovers {mostly smaller unbreakable particles of slates, silica sand and iron in traces} with David Leach porcelain:

And here I mixed sand of Mura river (river in northeast of Slovenia), which is not clay leftover but simple sand, into David Leach porcelain. From the look of it after firing it’s made of silica, mica and iron.

Do you like the speckels?