A lime kiln is a small building or scientific contraption where limestone is converted into calcium oxide, or lime. Lime is an essential ingredient in mortar, which is used extensively in construction. It is also commonly used as a fertilizer, and can help increase the yield on many different agricultural crops. Traditional lime kilns were small brick huts in which limestone was burned over a grate. More modern iterations are indoor tower chambers where limestone can continuously be heated and collected.
Lime has been an important part of most cultures since primitive times. Kiln ruins can be found on nearly every continent. There are three primary types of lime kiln: a heap kiln, a periodic kiln, and a continuous kiln. The majority of the lime on the market today is created in a continuous kiln. Such a kiln is more expensive and sophisticated than either a heap or a periodic kiln, but produces a better product and generates more predictable, controlled results.
Heap kilns are rarely used anymore. Most were designed to be temporary, and were extremely primitive in form — often little more than limestone set up into a burn pile designed for one-time lime extraction. The stones would usually be placed over a grate atop an open flame, and the finished lime scraped out of the ashes once the fire went out. This process was imprecise, and prone to inadvertent inclusion of ash and other contaminants. It was primarily used on-site at building erections and just outside of limestone quarries.
Periodic lime kilns are much more common. These are permanent structures designed specifically for lime harvesting. Nearly all are made of brick, often built several layers deep to provide insulation. Inside the kiln is a place for a wood fire. Above that, small chunks of limestone are stacked in a dome-shape. There is usually room for one or two people to stand and tend the fire and monitor the lime kiln operation, though once the flames get going, the room is usually too hot to be in.
A small hole, called the eye, is at the base of the kiln, and is where the finished lime builds up and is collected. The whole process typically takes several days. First, the stone must be heated, and then it must process, forming lime. Once the lime has been isolated and funneled out into the eye, it must cool before it can be handled and collected. Using a periodic lime kiln is generally more accurate than a heap kiln, but nonetheless is prone to some impurities and cross-contamination.
One of the only ways to ensure a pure lime byproduct is to use a continuous lime kiln. Such a kiln is a stand-alone structure, but unlike its brick predecessors, it need not stand outside. Most are permanent parts of science labs of manufacturing facilities.
Continuous kilns are typically shaped like tall cylinders and are heated with oil through a centralized furnace. Limestone must be loaded into the top, heated as it passes through the center, then ejected as pure calcium oxide at the bottom end. Using oil reduces the chance of ash or soot build-up, and the kiln can be continually fed as long as there is raw limestone to process.
Another advantage of a continuous kiln is management of fumes and lime dust. Lime dust is particularly corrosive, which is one reason that outdoor structures were normally built some distance from villages and other structures. The lime kiln process also emits carbon dioxide gas, which can be toxic. Most industrial kilns today have sophisticated means of capturing dust and diffusing carbon dioxide emissions so as to promote both user safety and environmental health.