I’ve built my furnace; mistakes were made along the way which I’ll document in a later episode. I decided on a 10 3/4 inch outer diameter with a 2 inch wall. The furnace is 15 1/4 inches high, and 2 1/4 of that is the lid. Thus the bore is a cylinder 6 3/4 inches in diameter and 11 inches tall. 

This took just slightly less than the complete contents of two 55-pound bags of “mizzou” castable refractory cement, which I obtained at Seattle Pottery Supply. Interestingly enough there were no instructions on the bag. Fortunately the (very informative) high temperature tools web site had detailed instructions, which I reproduce for you here:

• Material should be stored in a dry place. 
• Porous back-up materials or wood forms should be waterproofed. Absorption of water can result in reduced flow for the product. 
• Forms must be stout and water tight. 
• This product is designed to be mixed with water and then poured/handcast into place. 
• For best results, water should be maintained at 50-70F. 
• Approximate Water For Installation: 55 lbs. to 5 pints of water. 
• Mix for at least three minutes. 
• For best results, wet mix temperature should be maintained at 60-75F. 
• Minor adjustments to the amount of water are permissible to achieve desired flow. 
• Do not exceed 11.0% water under any circumstances. 
• Place material promptly. 
• Do not trowel to slick finish. 
• At temperatures above 60F, air cure, keeping surfaces damp and/or covered, for 16-24 hours typically or until a hard set has developed. Lower temperatures will increase the time before a hard set develops. The best results are achieved at curing temperatures of 90-110F. 
• Keep material from freezing during air cure and preferably until a dryout can be initiated. Freezing of this product prior to water removal can cause structural damage. 
• Never enclose a castable in a vapor-tight encasement as a dangerous steam explosion may result.

Typical dryout schedule for a single layer, 9” thick or less:

• Ambient to 250F at 75F per hour. Hold at 250F 1/2 hour per inch thickness.
• 250F to 500F at 75F per hour. Hold at 500F 1/2 hour per inch thickness.
• 500F to 1000F at 75F per hour. Hold at 1000F 1/2 hour per inch thickness
• 1000F to use temperature 75F per hour

I made a mold out of sheet metal for the inner and outer round surfaces, and plywood disks for the bottoms. The inner mold is held concentric with the outer mold by putting five or six two-inch pieces of wood around the circumference of the inner mold. As I mentioned in a previous episode, I soaked the wood in cooking spray, which was a convenient way to keep it from absorbing water.

The forces on the inner mold are going to be large when there’s eighty pounds of wet cement pushing on it, more than enough to collapse the flimsy sheet metal, so I filled the inner mold entirely with sand.  

I mixed up the cement by putting ten pints — just under five liters — of water in a watering can; this made sure that I did not accidentally put in too much water. I slowly added the water to the cement powder, stirring with a hoe. For easy cleanup, I mixed it in a bin lined with some scrap plastic sheeting.

I then scooped the cement into the mold and rammed it down with one of the wooden sections used to keep the molds concentric, going from one section to the next. I rammed it down pretty hard, and even still, there were a fair number of air bubbles in the finished product. This is not fatal, or even all that undesirable; air pockets are good insulators and lower the thermal mass of the furnace. The risk is that if water gets stuck in a pocket then it could expand and crack the furnace or cause spalling. Ram it a lot.

Once it was done I wrapped it up in plastic for a day while the hydrating reactions hardened the cement. Since the hardening reaction requires water it’s important that the edges not dry out too early.

Then I removed the molds, wrapped the whole thing up in a damp towel and more plastic, put a 60 watt light bulb inside, and left it for a week.

After that, I made some increasingly hot fires in the furnace. There was almost no visible steam at any point and no cracking, so I think I’ve got myself a furnace here.

Next time: however, some mistakes were made.