How to make IRC

Make IRC from Common Materials

The article that follows explains how you can make your own IRC (Insulating Castable Refractory) from common materials. This article was written by John Wasser. You can find other very interesting material on his site by going to I encourage you to check it out.

Making Insulating Castable Refractory from Common Material

Material you will need:

  • High Temperature Furnace Cement (hardware store)
  • Perlite (garden store)
  • A little water
  • A rubber spatula

Making Insulating Castable Refractory

It was easier than I thought to make insulating castable refractory for a foundry lining from material available from local hardware and garden stores. I got this idea from a web site about making a natural gas forge ( <> by Raymond "Swage" Maiara, Aurora Forge, New York, New York). The trick is to glue Perlite beads together with Furnace Cement (a.k.a. Furnace Mortar or Refractory Mortar). The material is slightly sticky and holds its shape well when packed. After it is set you can cut it with common woodworking tools.

The Perlite comes from the garden supply section of your local store and is used as a "soil conditioner". The price for a two gallon bag of Perlite was less than $3 in the garden section of my local hardware store.

The Furnace Cement comes from the furnace or fireplace section of your local hardware store. Look for Furnace Cement (or Furnace Mortar or Refractory Mortar) that says something like "Withstands temperatures to 3000¡F" and "Contains Silicates". It generally comes in "11-ounce" tubes (like caulk) and 32-ounce (1 quart) or 64-ounce (half gallon) plastic tubs. The price of a 32-ounce tub of: "Worcester Brush HIGH TEMP Furnace Cement (Black)" was less than $4 at my local hardware store.

You will need about 1 part (by volume) of Furnace Cement for each 4 parts (by volume) of Perlite so for a two gallon bag of Perlite you will need a half gallon of Furnace Cement. If you use much less than four volumes of Perlite for each volume of Furnace Cement all of the passages between Perlite beads will be sealed and it will take a long time for the cement to set (it needs contact with air). If you use much more than five volumes of Perlite for each volume of Furnace Cement the resulting material will be quite weak. You will also want to have some Furnace Cement to use as a sealing coat on your lining.

The Furnace Cement has about the consistency of roofing tar and is very sticky. It is MUCH easier to work with if you add about 2 cups of water per gallon of cement. This makes it more like a thin plaster. A rubber spatula is good for getting the cement out of the plastic tub.

You can mix a large quantity and keep it in a sealed container for a long time. A 5-gallon bucket is ideal for mixing a two-gallon bag of Perlite with 1/2 gallon of Furnace Cement.

Sealing the Surfaces

Once the C/P has fully set you can improve the insulating properties and make the piece stronger by painting some thinned Furnace Cement on all exposed surfaces. For this you may want to add even more water to the cement to make it flow better. Put on enough to fill any surface pores. The brush will clean easily with water.


I made a little foundry for melting aluminum and brass (See: Making a Propane-Fired Coffee-Can Foundry: <>). The foundry uses about 1-1/4" of this insulation lining a large coffee can. It has been up to aluminum melting temperature for over an hour and the painted label on the can has not charred! I would expect 1" to 2" of insulation would be enough for most any purpose.

One of the members of NEMES remarked that he was having trouble with a furnace made with this formula. The lining was slumping. I have since completed my own larger furnace (See: A Propane-Fired Foundry Furnace: <>). I lined it with steel and have not had any problems in the first two melts (aluminum and brass). Only experience will show if the lining will hold up.