Below is some data for both expanded vermiculite and perlite for use as insulations in either loose fill or mixed as a light weight concrete
For those who whish to experiment further with refractory insulations, try a 50/50% mix of clay and sawdust mould into brick shapes and fire to approx 1400°C
| Colour | Light to dark brown |
| Shape | Accordion-shaped granule |
| Bulk density | 64-160 kg/cu m 4-10 lb/cu ft |
| Moisture loss at 110 °C (230 °F) | 4-10% |
| pH in water | 6-9 |
| Combustibility | Non-combustible |
| Sintering Temperature | 1150-1250 °C 2100-2280 °F |
| Fusion Point | 1200-1320 °C 2200-2400 °F |
| Cation exchange capacity (b): | 50-150 me/100g |
| Specific Heat | 0.26 Btu/lb F |
| Thermal conductivity | 0,65 W / m.K |
Perlite insulation
| Color | White |
| Refractive Index | 1.5 |
| Free Moisture, Maximum | 0.5% |
| pH (of water slurry) | 6.5-8.0 |
| Specific Gravity | 2.2-2.4 |
| Bulk Density (normal) | 2-15 lb/ft3 |
| Mesh Sizes (normal) | 4-40 and finer mesh |
| Softening Point | 1600-2000°F 871-1093°C |
| Fusion Point | 2300-2450°F 1260-1343°C |
| Specific Heat | 0.2BTU/lb °F |
| Thermal Conductivity | 0.049 W/mK °F |
As an example the thermal conductivity of Styrofoam is approximately (W/mK) 0.027
The problem with a furnace is the amount of heat lost through conduction, radiation and through the exhaust outlet, if naturally aspirated burners are used the exhaust port has to be tuned the furnace cavity to attain a low pressure balance within the furnace, this will require a large exhaust port, and as such provide an easy path for the heat to escape because the unburnt gas and air enter the furnace at what ever atmospheric temperature existing at the time, are then heated during the burning process some heat is transferred into the metal. crucible, and furnace lining, the remainder is carried away in the exhaust gas which is much wanted heat
If forced air burners are used the same transference of heat takes place but as the exhaust port can be smaller and internal pressure can be higher more heat is transferred into the load before emitting out of the exhaust port, but care must be exercised to ensure adequate flame rotation or again wanted heat will be lost in the exhaust gasses.
The next major source of heat loss is conduction through the refractory walls and into the steel container and then radiated away. This is a serious problem and it is also a safety problem with a severe risk of causing injury, My furnace exterior is barely hand warm whilst operating at full temperature which is why I can raise the interior of the furnace to a good white heat.
A tip here is to use a rectangular slot as an exhaust port not a circular hole, as a slot forces the gas to change direction when exiting the furnace and hence slowing it down and consequently transferring more heat into the furnace.