In the operation of large solid-fuel boiler installations the fusion characteristics of the coal or coke ash are an important factor. If fusion and coagulation of the ash occur, trouble will be met in the raking of the boiler and serious blockages may result. Standard testing procedures for observing the fusion points of ash in the laboratory have been established for a considerable time, but a revised specification has now been prepared by the British Standards Institution and is shortly to be issued as BS 1016: Part 15: 1960.

Among other modifications, the type of furnace to be used for the determination has been specified more closely, and to meet this need an appropriately modified Johnson Matthey platinum-wound furnace has been designed and produced.

This furnace, which has a maximum operating temperature of 1500°C, contains two rhodium-platinum windings, each incorporating a built-in thermocouple. Either of these thermocouples may be used with a temperature indicator, preferably of the type incorporating over-temperature protection.

The furnace temperature will thus be registered on the indicator and the specimen temperature should be measured by observation through the window with an optical pyrometer.

An impervious alumina tube is passed through the furnace and extends at each end. Special gas sealing attachments are secured to the tube ends to permit the use of a controlled atmosphere inside the tube. The attachment at the front end contains a clear heat-resisting glass window through which the specimens can be observed, while the attachment at the other end of the tube is fitted with a bursting disc to ensure the safety of the operator should explosive conditions accidentally be created.

Gas is introduced into the furnace at the front end beneath the window and is discharged at the other end of the tube. Reducing conditions can be maintained in the furnace tube with a gas flow of less than half a litre per minute.

The principle of the test is to observe continuously a cone of ash as temperature rises at 5°C per minute and to note the temperatures at which three characteristic shapes occur. These are the initial deformation of the original cone, the formation of a hemisphere and the flow to a meniscus. As the initial deformation point is that at which fusion begins, and the other noted temperatures indicate the rate of subsequent fusion, the results indicate clearly the temperature above which serious fusion will occur.

Apart from the use for which it has been specially designed the furnace may also be used generally for controlled atmosphere work. In this case the built-in thermocouples will serve both for temperature measurement and control.