Heating items is a way of converting one item to another, or an item to a fluid. Items can be heated in many ways - in a Firepit, a Pit Kiln, or a Charcoal Forge, to name a few. However they all function in the same way. When you place items inside these devices, the items will gradually start to heat up. This is visible on the item's tooltip.
The temperature of an item is represented by a color, which will change through the following values:
Warming: 1 - 80 °C
Hot: 80 - 210 °C
Very Hot: 210 - 480 °C
Faint Red: 480 - 580 °C
Dark Red: 580 - 730 °C
Bright Red: 730 - 930 °C
Orange: 930 - 1100 °C
Yellow: 1100 - 1300 °C
Yellow White: 1300 - 1400 °C
White: 1400 - 1500 °C
Brilliant White: >1500 °C