|Small-Scale Brickmaking (ILO - WEP, 1984, 228 p.)|
|CHAPTER VII - FIRING|
Whatever system of firing is used, it is recommended that a low heat be first applied to the green bricks in order to drive off any residual moisture. This should continue until no more steam is evolved. This part of the heating is known as water-smoking. The completion of this firing stage may be simply tested by inserting a cold iron bar into a space purposely left between the bricks in the kiln, and by withdrawing it after only a few seconds. Condensation on the bar indicates that steam is still being evolved, and low heat should be continued until no condensation is found on the cold bar after re-insertion. This could take a whole week in some cases and needs plenty of air through the kiln.
Once water-smoking is complete, a rate of rise of temperature of 50 °C per hour may be safe in fully - controlled kilns, which are outside the scope of this memorandum. At the critical temperatures (e.g. quartz inversion) the rate can be temporarily reduced to avoid problems. In more simple kilns, the heating rate should be slower partly because of the lack of precise temperature control and partly because of the impossibility of getting enough fuel burning in some kiln designs. Although a slow rate of heating is safer, faster rates involve less heating time, lower heat losses and, therefore, lower costs. The optimal heating rate is that which requires the shortest heating schedule while yielding a product of satisfactory quality. A maximum of two weeks may be needed for the whole firing process.
Maximum temperatures with little air should be held for at least several hours. A whole day may be needed with some kilns with poor heat distribution in order to ensure a maximum yield of good quality bricks. During this firing stage, known as the soaking stage, the heat diffuses through the kiln, various chemical reactions take place and a glassy material is formed. Once the soaking is complete, the heat source may be removed.
The cooling rate should not be too rapid. In practice, natural cooling within the large mass of thousands of bricks, with limited air flow, is satisfactory. Cooling may take a whole week. More air may be allowed in once lower temperatures are reached in order to speed up cooling.