This old time foundry refractory mix for crucible or cupola furnaces was adopted by financially strapped foundry owners during the great depression, everyone suffered financial hardships during those days, so there was a lot of improvisation going on, now the backyard founder can save money making a home brew just like the old timers did.
First off, you need to make up a mixture of grog, i. e. slag free old broken firebricks, break them up into 50%-pea size & 50 % Walnut size. Place the grog mix in a large bucket or drum, and fill the drum with water until the grog is well covered; leave to soak until there is no sign of air bubbles.
Follow the next few steps carefully, as the amounts are made up by volume& not weight.
Take “three volumes" of the now very wet 50/50-grog mix and add “two volumes" of good fire clay. (The measuring volumes can be made up of anything from a small tin to a large bucket, depending on the quantity you may need) Mix thoroughly and temper it like moulding sand with a water glass mix of 50% water glass & 50% water. Water glass is the old name given for sodium silicate.
The final grog and fireclay mix should have about 6% moisture, it should easily form into a ball in your hands, if it's crumbly, it is too dry, and you may need to add more water. But watch carefully as you don't want to let it become like mud. Finally put your mix under plastic, and let it sweat & age for about 72 hours, or about three days. Now comes the part where you can actually begin to install the mixture in your cupola or gas fired furnace.
With the inner metal or wooden former in place, the mix can now be placed around the furnace wall cavity and gently rammed into place until the lining is complete. Smooth it of on top and then leave to air dry for a day or so.
Before you start ramming the mixture, to help drainage and ventilation later on when you cure the refractory, drill a series of 1/4" holes through the outer shell to help vent any moisture when the mixture is drying out. After a couple of days, you can light a small fire in the furnace and start to slowly dry & cure the refractory lining.
The drying stage is about the most important part of the whole process, and it needs to be printed bold. IF YOU DRY THE REFRACTORY OUT TOO FAST, IT WILL CRACK & SPALL, and ruin all your good work.
When the steam ceases from the vent holes, build a bigger fire up to where the melting bed or zone is. (10" to 14" from the bottom of the cupola Furnace) Turn the blower on, this will increase the heat and vitrify or glaze the rammed mix into a solid lining.
It is advisable to not use Borax while glazing the lining, borax is a flux and will have the effect of dissolving the lining, and you don't want that to happen. This home brew refractory lining you've made and installed in your furnace should never need replacing, except for the area around where the melting zone takes place. In which you may have to replace a small ring after every melt session is complete. In a gas fired furnace it will last for many, many years.
To find out more about hobby metal crafts and how you can employ the methods & techniques to build & create your own unique projects, visit our web site http://www.myhomefoundry.com to download a free ebook to begin your understanding of metal craft.