 Falling mortar
The refractory mortar between the elements of your oven shouldn't react to the steam from misting: This is a pretty standard baker's technique. If it's the right kind of mortar, it should have plenty of resistance to temperature shock to remain intact. I suspect that someone used cheaper mortar to build your oven.
You can point the oven from the inside: When it's cold, crawl in with the kind of carbide hook tool that they use for re-pointing tile in bathrooms, and scrape out any loose or crumbly mortar. Then mix a refractory mortar mix like "heat-stop 50" with water to a peanut butter consistancy, and squeeze it into the cracks. After a couple of hours, go in and sponge off any excess, and your ready to cure. A week to set, then small fires, and cool, progressively hotter, until you are back to baking temperatures.
I know that the down time might be a problem for a commercial oven that's fired consistantly. In all likelyhood, you don't need to do any more than scrape out the pieces that are in danger of falling into your food. Those things are pretty self-supporting.  |