The job actually included the entire scope of renewing the insulating cement, but making these patterns was far and away the most enjoyable aspect of the project.
The photos show the process, although perhaps not as clearly as would be ideal. I apologize for the random orientation- these were hasty cell phone shots taken on the fly.
Basically, the furnace shell is a steel cylinder about 40″ in diameter. It needs a 6″ thick layer insulation on the floor and wall. By itself, this is really pretty easy to accomplish by pouring a layer of cement on the floor and letting it set up. Then simply place a second, temporary, steel cylinder with 6″ of clearance all around it centered on the floor, and fill the space between the cylinders with more high temperature cement, subsequently removing the temporary cylinder.
The tricky part is that there needed to be three passageways penetrating the insulation: one for the one million BTU natural gas flame to enter the furnace, a second hole to act as a chimney, and a third hole set at floor level to act as an emergency drain for molten metal in case of a broken crucible.
These openings can’t be added after the refractory cement is poured and set; they’re created by putting specially shaped blocks in place prior to pouring. The blocks need to be shaped to allow them to be removed once the cement is hard, and most importantly, they need to leave a cavity of the proper shape once they are removed,
The furnace is an older unit, although the gas burner is a modern, so there was no literature available describing the shape of the transition from the burner to the interior of the furnace. All available experience was consulted, and a concept was eventually agreed upon. That still left me the interesting job of translating the concept into three dimensions, The final product, contrary to intuition, is actually the resulting cavity. All my thinking had to be approached as the negative shape of what I was holding in my hands.
The photos show the progression from defining the transition in a cardboard model to refining the shape from angles to curves, and finally seeing it in practice.
Improvements in the process became evident after the fact, a pretty typical occurrence. I can see now how to do the job more easily, and with better results. I wish I had a dime for every time I’ve said that,