A carefully graded powder is compacted as dense as possible with NO BINDER. On heating, the magnesia and alumina sinter to form the mineral spinel, and this expands and makes the lining tight. The cool part next to the coil remains powder, so that CRANKS CANNOT PENETRATE IT.
The powder is not rammed, stamped or vibrated, it is “spiked”. Dry spiking is done with a sharp pointed tool, which may have 3 or more spikes, typically 75mm long. The spikes push the material sideways more than down, and compaction progresses slowly from the bottom of the layer, and the spike penetrates less. The top 20mm does not become compacted. It gets compacted after the next layer is added. Dry spiking is a skilled job, which cannot be learned from reading instructions. It should be learned by working with an experienced team. If one small area is neglected, a leak to the coil can occur.
Place about 150mm layer of powder in the furnace. Spike it until the tools stop 20 to 30mm from the surface. This will take 6 men about half an hour. Add the next layer of 75mm and continue, up to about 30mm ABOVE the BOTTOM of the former level. Now scrape off the loose material on top, to the level of the former. Place the former in and start spiking the sidewalls. When the top is reached, dampen the powder with water and STAMP not spike, it down.
Cover with a capping material such as Keracap or Kerapatch.
The former needs to be about 6mm mild steel.
Place a “button” of solid steel inside the former, about half the weight of the furnace capacity.
Start the preheat slowly; about 10% power for the first hour, 20% power for the second hour, 30% power for the third hour etc.
Add scrap to fill the furnace once molten.
After 10 hours, at full power, the furnace should be at around 1650°C.
Soak for about half an hour at 1700°C.