I wanted to see what would happen if a solution of soda ash mixed with baking soda (containing equal weights of sodium carbonate and bicarbonate) was sprayed onto glazed pieces prior to firing at cone 6 in an electric kiln. You can see that the soda spray gives dramatic differences after firing. I found that there were no significant harmful effects of this treatment on the bricks and heating elements in the kiln.
Red-brown mug with no soda spray.
Red-brown mug with soda ash spray.
Green-brown mug with no ash spray.
Green-brown mug with ash soda spray.