On the other hand, firebricks used for furnace linings only melt at very high temperatures (and then they retract, or decompose, or become fracture-prone) and so have low fusibility.
To find the fusibility of certain compounds/materials, there are 2 methods: The most common test used to determine fusibility is to join two pieces with a variable heat source, increasing the temperature or power used in steps, and testing bond strength for each step.
If there is no step with good bond strength after joining, then fusibility is said to be low.
The values are determined using an empirical method based on the geometrical changes upon heating to a relevant temperature, of a conical ash sample.
The fusibility of the ash is of interest because it roughly indicates the properties of the clinker that will be produced during burning.