Masonry Magazine February 1971 Page. 22
82 Varieties
Ordinary manual firing, even with the closest watching and regulation, would be doing exceptionally well to maintain temperatures within plus or minus 20 to 25°F. of specification or a variation range ten times as great as the control of natural gas allows.
Essentially, there are six variables which determine brick color, explained Mr. Finzer. The first three: clay composition (i.e., percentage of shale or red to fire clay-or buff); size of the clay particles; and additives such as metallic oxides or other colorants are all ingredient-related functions. The formulation process controls these factors.
The other three, however, all concern the maturation process. Firing time and temperature constitute the most important element, with shorter time and lower temperature yielding lighter colors and larger brick (less shrinkage), while longer time and higher temperature produce the opposite effect, namely darker colors and smaller brick, because of greater shrinkage.
Next among the processing factors bearing on color of the finished brick is atmospheric composition after maturing temperature is reached. Oxygen-rich atmosphere in the curing oven tends to lighten color, so for the more intensive colors currently favored by architects and owners, an oxygen-poor atmosphere is preferred. This is created by introducing excess combustibles in the form of natural gas, to displace the normal concentration of oxygen in the kiln's interior atmosphere.
Moisture content of the firing atmosphere is also thought to affect color of the finished brick, although studies are still in process to establish this conclusively. With natural gas fuel, the requisite control over atmospheric moisture content within the tunnel kiln is said to be significantly easier to attain and maintain.
With three Swindel-Dressler tunnel kilns (representing an investment of better than $2 million) to process the equivalent of 1,500,000 brick per week, Mr. Finzer says, "Natural gas was the only logical choice for energy in the volume and quality we need. An operation of this size and exactitude simply would not be possible with solid fuels, even if we kept 20 men busy stoking and cleaning the kilns. And liquid fuels would never provide the clean combustion this process demands, not to mention the extra costs we'd have to pay for storage, handling, and pre-treatment facilities. No other fuel we know of stays so consistently uniform in Btu content, so that batch-to-batch variations don't have to concern us."
Mixed and blended ingredients for each particular variety of brick emerge from a steam-heated extruding die which imparts the proper cross-section to a continuous ribbon of hot, wet clay, much like the way toothpaste is forced out of its tube when you squeeze it.
The continuous ribbon of newly blended clay is cut on a traveling slicer, whose fine wires rotate 90 degrees on a cylindrical frame for each stroke. This action may produce 15 to 28 brick depending on specific length desired.
A horizontal belt conveyor then takes the formed and cut brick blanks to a stacking area, where the blanks are removed and stacked on kiln cars in such a manner as to provide free circulation of air to and around each surface. The kiln cars are then transferred automatically to the pre-drying ovens, also Swindel-Dressler units, heated by the exhaust from the three tunnel kilns.
Emerging from the pre-drying oven after a cycle that can range from 48 to 70 hours at temperatures up to 350°F., the cars of unfired brick are in turn stacked on transfer cars, electronically controlled to feed them into the tunnel kilns on remote signal from the central control console. This signal is energized automatically each time a car of finished brick emerges from the tunnel kilns discharge end.
Automatic controls regulate temperature in each of the tunnel kilns' five zones, gradually raising temperature to the requisite level by easy stages, to guard against potentially damaging thermal shock, or undesirable color patterns as well as unacceptable dimensions that could be caused by inadequate or excess shrinkage.
Pyrometric sensing devices in each firing zone monitor inside temperature and atmospheric content continuously, recording their readouts on disc or strip charts in the central control room. Any necessary adjustments in firing rate are also made automatically, so that temperatures stay within programmed specifications for each zone.
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