Santa Cruz County History - Making a Living

The Laguna Limekilns: Bonny Doon
by Robert W. Piwarzyk

[This article is excerpted from a manuscript titled, "The Laguna Limekilns: Bonny Doon," pp. 44-45. The maps, drawings, and photos of the manuscript are not included on this site. The manuscript is copyrighted 1996 by the author. It is used here with permission.]

Kiln Construction

Since ancient times, limestone has been burned (calcined) in open pits or piles with the fuel mixed or layered with the ore. Mud, or clay, was sometimes plastered on the outside to help contain the heat. Ventilation is needed to bring oxygen to the fuel and to remove the carbon dioxide and water vapor that is released. Draft holes around the bottom and an open top provided airflow. The next step was to line the pit with rocks to reflect the heat. This method is referred to as a "single chamber kiln." Such a kiln had to be loaded with a proper amount of fuel, since there was no way to add any, and the ashes ended up being mixed with the lime. It is better to keep them separated to prevent this from happening, and so the "double chamber kiln" came into use. Another advantage to this design is that fuel can be added as needed. Separation has been accomplished by various means -- rock or metal grates being the most common.

The second chamber (or firebox) was created by constructing a tunnel in the limestone as it was loaded into the kiln. This was done by the "archer," who piled the limestone ore to form an arched tunnel from the front access door to the rear wall of the kiln. The surface of the tunnel acts as a grate if voids are provided for the flames to pass through. The kiln is in effect an updraft furnace or oven. A rock grate can be the ore itself, but when limestone calcines, it tends to crumble. The higher the grade of ore the more it does, so low grade ore was used to build the arch. Such crumbling is disastrous if it causes the arch to collapse. After the fuel burns out and the load cools off, the kiln must be emptied, repaired (if needed), and reloaded.

To provide access to the fuel chambers, openings were constructed through the lower part of the kiln wall. The kiln was either built free-standing or into a hillside with the downhill wall exposed. The latter method was easier to construct as the earth supports three of the walls, the back and sides. The front wall was therefore usually thicker and was buttressed. The shape of the kiln varies from circular to rectangular or oblong. The walls may be straight or flared (top opening area is less than the kiln floor area) and the corners may be squared or rounded. Flared kilns with rounded corners resemble a pot and are known as "pot kilns." This construction is the most efficient as the heat is reflected inward and distributed evenly. There is less chance of the limestone being overburned or underburned. Overburned ore was not used by lime producers on the West Coast, but was used in England to make a special cement. Underburned ore can be reloaded, but time, manpower, and fuel is wasted.

Loading was done from the top with the ore placed from bottom to top. Unloading was done from both the top and through the access doors at the bottom.

With larger kilns, more fire chambers were needed to provide even heating. Therefore more arches were required in the front wall, resulting in a greater span between the buttress walls. Single, double, and triple arch kilns were built. The larger producers all used kilns with four arches and these were constructed with three kilns to a facility.

Some kilns had a roof over the work area in front of the access doors. This protected the lime from the rain when the kiln was unloaded and placed in barrels. Quicklime is very unstable and becomes hydrated when water is added. Rain falling on the open top was also a problem. Wet fuel and muddy, impassable roads were other reasons that lime production ceased during the winter months.

The purer the limestone, the higher the temperature needed to burn it. The lowest temperature that pure calcium carbonate will calcine is 1648 degrees F (900 degrees C). Kilns were usually fired around 2200 degrees F. The draft was regulated by laying flat pieces of limestone or sheet metal over the top. Metal doors were also used on later kilns.

The temperatures and pressures involved put great stress on the walls, especially the front wall, which did not have the earth to help support the outward thrust of the loaded ore. In addition, if the wall was constructed of limestone, it also would turn to lime which weakens the wall and becomes a problem the older the kiln gets. The arches in the thick front wall tend to trap the heat and are especially vulnerable. Therefore, firebricks were used to line the arches and the kiln walls, and were often double layered. Regular building bricks vitrify (deform and turn glassy black) at these high temperatures and do not make as good a lining as firebricks. Metal arch inserts were also used and have been found at two sites. These are either forged or cast iron. A lining, made of firebrick with refractory mortar, not only reflected the heat but kept the quicklime from adhering to the wall, making unloading easier. When used on the fire chamber floor, it also provided a surface which made it easier to shovel away the ashes.

Even thick front walls were subject to cracking and bowing, and one kiln at Grey Whale Ranch has a collapsed front wall. These failures were sometimes prevented or reduced by adding shoring or by buttressing the wall. Many kilns were constructed with built-in (integral) buttress walls. At several sites log shoring and/or stone walls were added in three places (between each arch) as the wall weakened with age, and at one a stone buttress wall was added to the original structure.

A single-pot kiln does not use labor effectively. Loading, burning, cooling, unloading, and repairing are the steps for manufacturing quicklime from limestone, and with a single kiln these steps must be done in sequence. A double-pot (side-by-side kilns) allows the crew to load and unload a kiln while the other kiln is burning or cooling. Adding a third pot enables a crew to always be unloading one kiln, while another crew is always loading a second, and the stokers are always tending the fire in the third. With this specialization, two stokers each worked a 12-hour shift providing around the clock burning. When one load was done "cooking," the next load would be ready to start and so they moved from one kiln to the next. A work cycle for one kiln varied but generally it took 1 to 2 days for loading, 3 to 5 for burning, 2 for cooling, and 2 for unloading. A day or two was usually needed for repairs on the fire mortar or brick lining on the inner walls. Thus a burn averaged 9 to 13 days. Loading or unloading a larger kiln took more men, not more time.

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