According to "Marks' Standard Handbook for Mechanical Engineers", 10th, coal gas burns at about 3,590°F (1,977°C) under 100% air conditions. More or less air will decrease the temperature. This means that the maximum temperature of a coal fire in a forge is about 3,500°F (1,927°C).
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Most organic compounds have a constant-pressure adiabatic flame temperature in a narrow range around 1950 °C. Because compounds like wood, fat, and petroleum all have C-H bonds, C-C bonds and O-O bonds that are broken to release energy in roughly the same ratios, the adiabatic flame temperatures for these substances are nearly indistinguishable. It should be noted that, these temperatures are vastly higher than what any thermocouple inserted into a fire will register!
This means you will be able to melt common metals such as:
| Metal | Melting Point in °C |
| Brass | 930 |
| Copper | 1084 |
| Gold | 1063 |
| Iron | 1536 |
| Lead | 327 |
| Stainless Steel | 1510 |
But not metals such as:
| Metal | Melting Point in°C |
| Tungsten | 3400 |
| Molybdenum | 2620 |
| Iridium | 2450 |