I had been using my Dross for high-purity Rock Maracas, but they don't seem to be accepting Dross as an ingredient any more since the last update.
I've also used my dross for Filet on the Rocks, which given that it's 2/3 rock results in a good amount of purity being maintained. Haven't checked if I can still use Dross for that, since I'm out of fish.
I've thought about using Dross on my fields for a speed bonus, but the plenty penalty makes me leery of doing that.
With 20+ dross produced with each smelter operation you have access to an abundant source of rocks with a purity bonus, but it's SO abundant I find myself just dumping most of the dross, keeping only the higher purity stuff from lime-free batches and storing it in construction signs. However, I now have over 100 units of 13% purity dross in storage, and I feel bad throwing it away. Is there something productive I can use my dross for that I haven't thought of?