[Alternating Soda/Salt Firing in the same kiln ?]

16 minutes ago, Callie Beller Diesel said:

Salt is mildly more corrosive to kiln brick than soda is, but the difference is somewhat academic. You might get a few more firings out of a kiln that’s dedicated to soda, but you’d still have to do replacements/repair work within a comparable number of firings. This based on the observations of the soda and salt kilns at Medalta in Medicine Hat, Canada. Those kilns are both outdoors, but under a metal shed roof with walls that offer protection from the winds and precipitation, but not temperature. They use the same kaolin/alumina kiln wash, and offer a couple of wadding recipes in both. Exposure to the elements and a bit of procrastination can actually assist with soda firing cleanup if you’re using Gail Nichols’ fine soda/whiting/water mix: the firebox residue will crumble within a day or 2 depending on humidity and can be swept rather than chipped out.

The reasons that you’d want to have a soda only kiln have more to do with the end results on the pottery, as I think they found chlorine emissions from a pottery sized salt kiln were less than an outdoor swimming pool. That said, you should still wear respiratory protection when adding your sodium of choice, because both are caustic and are bad for soft little alveoli at close range. 

Salt vapourizes readily in the heat of a kiln. Soda is a lot more sluggish, which is why you need to dissolve it in water to get it to travel on a kiln draft. That means soda’s inclination is to leave more directional marks from the flame path rather than salt’s more overall coverage. If you really want to create directional marks, skip the sprayer and use the aforementioned “plaster” mix, or one of the many burrito methods. 

Soda gives a somewhat different colour response when used by itself, and leans towards cool/grey tones in places where it builds up. It can give a more crystalline/opaque surface where it gets really thick, especially if you watch your cooling. Salt seems to lean towards more brown, and seems to remain quite glossy. Some of this is going to depend on your reduction cycles, clay bodies, cooling cycles and a bunch of other factors, so what I’ve mentioned are more a generalization more than hard and fast rules. 

If people want to work with a soda specific aesthetic, which can be quite different from salt, you have to keep salt out of the kiln you’re using. Any significant salt buildup is going to affect future firings due to that volatility. 

If you’re looking to replicate a salt fired look but substituting soda ash for salt, contamination may not be a big concern for you. Residual salt can help reduce dry spots if you don’t want to be adding a whole bunch of soda ash, or if you’ve got beginners or people who may not be able to refire pieces. 

Thank you so much for your thoughts! We will check out that recipe  - I couldn't think of a reason why alternating occasionally could be a problem technically, and I think its pretty desirable for us to be able to do some experimentation with this particular kiln. Thank you again!

[Alternating Soda/Salt Firing in the same kiln ?]

Hello! I work as a studio technician at a community studio. We are currently rebuilding a (previously Salt) kiln and are planning to switch it to be a soda kiln (adding additional ports, ect) - the studio manager and I have been toying with the idea of mostly doing soda firings, but occasionally offering a salt firing every now and then. I understand when a kiln becomes a soda/salt kiln, the bricks hold the build-up and hence, once salt, always salt... Our curiosity is if there is any trouble or additional considerations with switching between the two processes, so long as the kiln is designed to accommodate both? Some specific questions are:

Should the wall/shelving wash be focumalted differently for both firings?

Would introducing sodium in both methods over time cause more wear and tear on the kiln rather than sticking with one process? 

Are there any other things we should be considering when thinking this through?

Additional details:

We plan on building a ~30cu/ft cross draft kiln in a sprung arch design and would plan to use a pressurized sprayer to introduce the soda sprays. The kiln is outdoors and we are in Michigan so it gets pretty worn by the weather. We are looking to have more control over more subtle directional glazing affects than the salt kiln previously had, so soda seem ideal in terms of controlling the saturation of the sodium, hence the desire to change.  Thank you!