The behaviour of the coals ash residue at high temperature is a critical factor in selecting coals for steam power generation. Most furnaces are designed to remove ash as a powdery residue. Coal which has ash that fuses into a hard glassy slag known as clinker' is usually unsatisfactory in furnaces as it requires cleaning. However, furnaces can be designed to handle the clinker, generally by removing it as a molten liquid.
Ash fusion temperatures are determined by viewing a moulded specimen of the coal ash through an observation window in a high-temperature furnace. The ash, in the form of a cone, pyramid or cube, is heated steadily past 1000 °C to as high a temperature as possible, preferably . The following temperatures are recorded;
* Deformation temperature: This is reached when the corners of the mould first become rounded.
* Softening (sphere) temperature: This is reached when the top of the mould takes on a spherical shape.
* Hemisphere temperature: This is reached when the entire mould takes on a hemisphere shape.
* Flow (fluid) temperature: This is reached when the molten ash collapses to a flattened button on the furnace floor.