In my most recent batch of pots I’ve had some problems with “Scumming”, water soluble salts coming to the surface sometimes in drying and other times when gazed firing. It will leave a bumpy rough surface which is not very attractive. One of the mysteries is that it seems to happen , not to all pieces but to a series of pieces.
In doing some research I came across this article by Jeff Zamek https://www.thefreelibrary.com/Soluble+salts+in+clay%3a+Jeff+Zamek+details+causes+and+solutions.-a0347971489 one suggestion was a clay body with a lot of bentonite may have this issue ( I use a porcelain body) Another possible cause the article suggested was too much deflocculant such as Darvan 7 could add a soluble salt to the clay. I’ve always added a teaspoon of Darvan 7 to my water and then added my dried clay scraps and mixed it up. Never really noticed a problem. But recently what I have done is when my throwing water is getting thick I add a teaspoon of Darvan 7 and then addition scraps of clay. Use an immersion blender and making almost a casting slip pour it in my drying slab and recycle it. I feel that may cause of too much soluble salts in a small batch of recycle clay. The article also recommended adding up to 2% Barium Carbonate to possibly reduce the effects of the soluble salts.
I will just add my throwing water slop to another bucket and add water and barium carbonate to this mixture when I recycle a bigger batch of clay. I will follow up in a future See you in the next post
link below to drying slab