I have used a soda ash wash and it gave a nice, low luster, finish, while maintaining the red clay color at Cone 6. I made mine by dissolving the soda ash into the hottest water coming out of my faucet, stirring constantly while dropping the powder into the container until a few crystals wouldn't dissolve. I decanted and stored the rest in a non-reactive container. I have not noticed it precipitating out (which is why I assume Chris recommended making only what you use for one time.) The result is caustic, so please be careful with the liquid, and particularly don't get it in your eyes.
You can search at Ceramic Arts Daily and get more information. It turns up several discussions in the forum of this subject.