Heavy clay products such as bricks tend to be stacked on top of  each other on kiln cars and fired. The resulting force on the lower layers becomes significant and the firing under load properties may significantly effect the pack stability during the firing process.

In simplest terms when materials start to expand during heating (most brick making clays tend to expand between 0.5% and 2% during the early part of the firing process, this on a 2m high this equates to 1 to 4 cm increase in size) they may cause the packs of bricks to ‘flower’ out near the top, resulting in bricks distorting and cracking during firing. In rare cases the outer bricks may catch the kiln sides and create a kiln wreck.

The shrinkage tends to be between 2 to 12% over the vitrification range, here the gradient and shape of the vitrification range is important.

Typical Firing under Load plots

 Coal Measure Shale

As a general rule a low gradient around the preferred firing temperature is ideal as slight variations in the production firing profile can be used to minimise the range of fired product size variations.

The technique is ideal for assessing unknown materials as the ideal firing temperature can be estimated from looking at the curve.

With materials already used for production, it is possible in conjunction with differential thermal analysis to reduce the firing time for the product. A series of firing under loads can be run, standard method, one that matches the current production firing profile and then a series of modified profiles. The project aim can be to reduce certain faults that may arise during firing or to speed up the firing process. Alternatively, the clay body may be modified to change the firing characteristics. Adding an Etruria marl to a keuper will make the keuper body easier to control.

The Churchill Laboratories in-house standard test method involves plastic pressing small bars

(approximately 3x4x11 cm long ). These are allowed to air dry overnight before placing in a dryer for a minimum of 24 hours at 110 ^oC to thoroughly dry.

Six of these bars are placed in the kiln in a stack two on two and a load of approximately 34 N/mm² is applied to the top of this setting. The briquettes are fired at a rate of 1 ^oC per minute rise in temperature up to a maximum of 1150 ^oC or to a maximum designated by the customer. Whilst the firing is taking place the linear change in height of the setting is monitored using a linear transducer. The outputs from the transducer and the thermocouples are recorded in a data logger.

The information logged is down-loaded on to computer and processed to produce a plot showing linear size change against temperature.