Last time we discussed how the furnace body material needs to have a low thermal conductivity, to ensure that temperature builds up inside the furnace; this has the nice additional property that the outside of the furnace remains relatively cool, at least in the non-steady state.
The material also needs to have small thermal expansion. Because the thermal conductivity is, by assumption, low, there will be a large thermal gradiant; that is, when it is fired up, there will be areas of the furnace body that are very hot, and areas that are relatively very cold; if the body expands significantly more in the hot sections more than the cold sections then we have a thermal shock scenario, which can fracture the furnace.
A substance which has these properties is said to be refractory; I’m going to make my furnace out of refractory cement.
What is so special about refractory cement? Why not use ordinary Portland cement, or even concrete?
Cement works by undergoing a chemical reaction in the presence of water that essentially causes it to crystalize. Doing so can trap considerable amounts of water in the body of the cement. Concrete is essentially a mixture of cement and hunks of rock. When these substances are used in a high-temperature application, the trapped water will attempt to vaporize and form a high-pressure steam; if the pressure gets high enough, cracks can form explosively. And concrete may also contain rocks that fracture under heat, which can make the situation worse. Do not use ordinary cement or concrete.
Refractory cement is typically ordinary Portland cement plus additional chemical additives which encourage far less water retention in the cement. But since the chemical reactions that make the cement crystalize in the first place are hydrating reactions, there’s got to be enough water in the cement throughout the curing process to ensure that it hardens throughout.
So, some important tips for casting refractory cement:
- Cement is caustic when wet, presents an inhalation hazard when dry, and undergoes an exothermic (heat-producing) reaction when curing. Keep this in mind and use the appropriate safety gear.
- Use the entire bag. As the bag was shaken on the truck from the factory, it might no longer be a consistent mixture of the necessary chemicals.
- Mix in exactly the amount of water recommended by the manufacturer. When it is adequately mixed you should be able to make a snowball out of it and throw it without it either liquefying or crumbling as you do so.
- If using a wooden mould, spray the mould surfaces that will get cement on them with cooking oil spray. This will discourage the cement from losing water too quickly into the wood as it cures.
- Cement is extremely strong in compression but has poor tensile strength. Bend some steel coat hangers or rebar and use it as reinforcement inside the concrete, particularly in the lid.
- Avoid the creation of air pockets; they will contain air which can also expand when heated and encourage cracks. Get a hammer drill and a piece of scrap wood. Use the vibration of the hammer drill against the wood to vibrate the surface of the cement; this will drive out the bubbles.
- Do not wet the surface and trowel it smooth. Smooth it out with your (gloved) hands.
Once it is cast then the curing process begins. It is very important that the cement have the right water level as it cures; the cement should be very dry when the process is done, but it cannot dry too quickly otherwise the hydrating reactions that make it strong will not have time to take effect. Also, you don’t want to be in a situation where the surfaces have hardened so much that they are trapping lots of water inside.
- For the first 24 hours, keep the exposed surfaces covered with plastic sheeting or damp rags; the edges will dry first but they need to stay damp so that they harden.
- After the first 24 hours, put a 75 watt light bulb (lit!) inside the furnace. This will provide enough heat to slowly drive much of the excess water out of the cement. Leave it there for a week or so.
- The last, and most crucial stage, is the calcining stage, when the last of the water is driven out and the final chemical reactions take place. Build a small fire in the furnace and bring it up to the metal-melting temperature over a period of many hours, the longer, the better.