Asphalt was made from the mixture of certain proportions of aggregates with bitumen which produce a homogenous mixture with certain percentage of materials and temperature criteria. Before mixing, aggregates should be mixed together to make the required aggregation. Then to remove the moisture, aggregates heated in required temperature to be coated by bitumen.
The first method of producing coating materials was using a shallow tray placed on a coal stove. Broken aggregates and almost aggregated, heated on trays and after drying, bitumen added to them and mixture was stirred manually. Mechanical machines were developed in the late nineteenth century. These machines were made based on concrete mixers including columnar dryers by coal flames to dry and heat aggregates. Then Pag-mill or oar mixer which have an axis with multiple blades were developed that some of them were used until the 1920s.
Three different dryers were developed in UK:
1. A long rotating cylinder
2. Columnar dryer which materials slowly moves downwards while the resulting gases pushed upwards.
3. Short modular cylindrical dryer which is the prototype of modular heating mixer.
In the early of 1930s and in the late of 1940s, oil was replaced coal as a heating fuel. Initially, the hot gases from the torch were driven into the cylinder. But with the advancement of technology, the torch was taken into the cylinder and it still being used in this way.