Bricks were fired in kilns to remove volatile substances (such as water and carbon dioxide), leave the clay more porous, shrink the mass through heating, and harden the brick by fusing the clay particles. The kilns were either up-draft or down-draft, depending on which way the heat was transferred to the bricks. Up-draft kilns take the heat from the fireboxes and transfer it up through the bricks to the chimney at the top. In down-draft kilns, the heat is transferred from the fire box to the top of the kiln by flues and then out the bottom of the kiln to the stack.
The simplest kilns were the "scove" kilns. These were temporary structures often used for the firing of common brick. This type of kiln could not reach the higher temperatures possible in a permanent kiln. The bricks were set in a large rectangular mass with a series of arches left in the bottom. Then they were surrounded by a wall of "double-coal" brick which was covered with wet clay to prevent the entrance of cold air. Then the top of the kiln was closed by a layer of bricks. Dutch Kilns were similar to the scove kilns except they had permanent walls to prevent entrance of cold air. Almost all of the front brick plants and many of the common brick plants used kilns that had permanent walls, roofs, and entrances at either side. These kilns were much more reliable, had better heat regulation, and could achieve higher heating temperatures. In 1904, about 70 percent of the brickyards in New Jersey were using permanent kilns. Continuous Kilns consisted of a series of chambers in a line, circle, or oval, that were connected to one another and a stack by flues. The heat from an initial firing could be transferred to another chamber in order to utilize waste heat. The kilns were usually fueled by coal, and occasionally by wood (see some workers loading coal into a kiln).
At the S & F Company, the clays were red-burning, nonrefractory, and had exceptional strength if fired slowly in the initial stages of firing. The common brick clay was micaceous and sandy. It had a tensile strength of about 75 pounds per square inch. The hollow brick clay was dark gray, fine-grained, and had a tensile strength of about 84 pounds per square inch. The hollow brick clay was mixed with other local clays for best results. The different shades of brick were accomplished by the addition of artificial coloring agents and manipulating the temperature of the kiln fires (Ries and Kummel, 1904: 239-241, 247, 470). At S & F Company, the different shades of brick were attained by manipulating different clay types without the addition of artificial agents (Sayre & Fisher, 1895). Investigations by LBA in 1990 at the S & F Company may have discovered the remains of a rectangular permanent kiln as large as 100 feet by 125 feet. (PCI, 2002: 2-34 and LBA, 1990: VI-10-11) (see an example of a permanent rectangular kiln). Historical map research done by PCI in 2002 indicated a group of buildings that may have contained circular permanent kilns, one of which was marked on the 1931 Sanborn map as having a diameter of 25 feet (PCI, 2002: 2-30-36) (see examples of permanent circular kilns) (see also the archaeological investigation section).