Lime Kilns


Lime Kiln Monacacy Broadwell 2012
Lime kilns were (and still are) used to convert anything that contains calcium carbonate (CaCO3) into quicklime by adding energy in the form of heat


Lime is a generic term that can colloquially refer to any of a variety of compounds that have calcium as their primary constituent. More properly lime is calcium oxide (CaO), which is also known as quicklime, burnt lime, lump lime and unslaked lime. The latter term is to distinguish it from slaked lime – the addition of water to quicklime to produce hydrated lime or calcium hydroxide (Ca(OH) 2). The word lime is of antiquated provenance, surviving as lim in Old English from its original Proto Indo-European root slei which meant sticky or slimy (slime was originally moist earth; its homonymous association with lime is not coincidental). The original meaning was then something that had a stickiness property – this suits one of lime’s primary applications as the glue that holds cement together. It is also the reason for the use of the term birdlime for a very sticky substance made from a variety of constituents applied to tree branches in order to capture birds. Birdlime contains no lime; it means bird-stick according to the etymological root.

Lime kilns were (and still are) used to convert anything that contains calcium carbonate (CaCO3) into quicklime by adding energy in the form of heat to separate the carbon dioxide (CO2). The chemical equation is CaCO3 + heat -> CaO + CO2 where heat is on the order of 3 mega joules (MJ) or about one kilowatt-hour (KwH) for every kilogram (KG) of lime produced. The temperature required to generate this amount of heat is about 1700°F (900°C). Calcium carbonate is one of the most abundant compounds on earth. It is the primary constituent of limestone – a portmanteau word that conveys the association of being a lime-bearing stone. Limestone is formed in ocean basins by the buildup of the calcareous remains of sea creatures, notably shell-producing mollusks, in the sediments. Over many millennia, the sedimentary layers form sedimentary rocks due to the compression of overlying deposits. Limestone constitutes between ten and twenty percent of the earth’s sedimentary rock; there are major deposits in the form of ocean-sized areas on every continent. Lime kilns used to convert limestone to quicklime were located according to necessary proximity to the raw materials of calcium carbonate limestone and wood or coal for the generation of heat.

The etiology of lime production using refractory materials is generally attributed to the Romans, whose monuments, buildings and roadways were the fons et origio for the magnificent constructions of Western Europe; the Egyptians, Greeks and Asians used stone. Stated simply, the Roman Empire was built with Roman cement, consisting of a mixture of hydrated lime (calcium hydroxide) and volcanic ash readily available on the tectonically active Italian peninsula. The discovery that heated limestone produced quicklime was certainly accidental, the likely result of a very hot campfire on a bed of limestone to which water was added as an extinguishment – to form cement. The simplest type of lime kiln, known as a clamp kiln, emulates this arrangement. It consists of a pile of limestone interlaminated with layers of either wood or coal and covered with clayey earth to encourage a slow, high heat generation. At the end of the burn, which could easily take a week, the pile was exhumed and the powdery quicklime removed, albeit comingled with ash and other detritus. The Romans improved on this rudimentary arrangement with the progenitor of the lime kilns of Roman Britain (43 to 410 CE) and subsequently, due to the British diaspora (of 1607 to 1776), to colonial North America.

The lime kilns of Great Britain were introduced by the Romans; there is no evidence of their use by the preceding Celtic population. Their use was primarily for the manufacture of lime materials for the construction of forts and villa-type edifices as evidenced by the location of kilns near to the ruins of these sites. . Due to the economics of raw material procurement and transport, lime kilns were necessarily located close to limestone quarries and either coal mines or forests. The use of kilns fell into desuetude with the departure of the Romans only to be revived in medieval Norman England (1066 – 1485) for the castles and churches of that era. The concomitant and subsequent use of lime for agricultural purposes may stem from this period. The lime kiln of England that became the lime kiln of New England was a simple affair. Called a flare kiln, it consisted of an open-top enclosure of refractory, granitic stone perforated with several openings at the bottom with an internal ledge. The limestone was stacked on top of the ledge and wood or coal was placed in the bottom. The openings, known as flues or stoke-holes, allowed the air of combustion to enter the hearth region. After about a week, the limestone having been calcified, the quicklime and ash were raked out of the hearth region for processing into construction materials. Due to the physics of the heat transfer necessary for the conversion limestone rock into quicklime powder, the flare kilns were all about the same size – producing 25 tons of lime with 50 tons of coal every week. Modern kilns are continuous process with a constant flow of crushed limestone and an output of up to 500 tons per day.

Quicklime is labile, preferring the lower energy state of greater stability. The natural carbon dioxide in the air suffices to restore calcium oxide to its calcium carbonate limestone natural state over time. The time frame for this reaction is slow enough to allow storage and use of the material without hermetic measures. However, water is a problem. The spontaneous exothermic reaction of one liter of water with an equal volume of quicklime according to CaO + H2O -> Ca(OH)2 or calcium hydroxide results in about 4 mega joules of heat. The material safety data (MSDS) sheet for quick lime offers the cautionary note that “addition of water to quicklime has generated temperatures as high as 800 °C. Some reports describe the reaction as violent.” To alleviate this problem, quicklime is frequently hydrated to produce slaked lime, or calcium hydroxide. In either case, quicklime and slaked lime are the primary constituents for a variety of fundamental construction materials including cement, mortar and plaster. There are a number of other uses, some of which are of significant cultural interest. Whitewash (alternatively lime-wash), as employed by Tom Sawyer in the inimitable Mark Twain subterfuge, was used not only to whiten surfaces, but also to waterproof them; the curing of the slaked lime whitewash with the absorption of carbon dioxide formed a protective calcium carbonate shield, a shell-like covering that was as impervious to water penetration as a clam. Lime was also used to bleach paper, clean the hair off of hides for tanning and as a disinfectant.

One of the more interesting cultural artifacts associated with quicklime is in its purported use as a means of dissolving animal carcasses , notably human bodies, so as to stymie the homicide investigator for lack of a corpus delecti. An oft quoted verse attributed to Oscar Wilde in an 1898 description of the burial of a Charles Woodbridge in a casket filled with quicklime attests to the pervasiveness of this belief:

Eats flesh and bones away
It eats the brittle bones by night
And the soft flesh by day
It eats the flesh and bones by turns
But it eats the heart away

It is not known where this belief arose, as the logic is specious. Acid (low PH) dissolves organic materials and lime is most assuredly basic (high PH), and therefore would, if anything, preserve the remains. And this is apparently exactly what it does. A 2012 study used pig carcasses to provide some empirical proof (pig skin is very similar to human skin). Two (obviously dead) pigs were buried with quicklime and two without quicklime for a period of six months. Exhumation revealed that the pigs without lime had been reduced to bones and the pigs with quicklime were largely intact. What quicklime is good for is to prevent the decomposition and putrefaction of dead animals (and people) by creating an environment that is inimical to decomposing bacteria. In fact, the Red Cross Emergency Relief Items Catalogue includes “LIME, QUICK, calcium oxide, powder, 5 kg box” for use “on corpses: If deep burial is not possible, first spread quick lime in the pit, then place the corpse and cover it with quick lime (1 kg of lime per 10 kg of corpse weight).” In 1874, Londoner Henry Wainwright did in his alleged mistress Harriet Lanes and packed her remains in quicklime. A year later, while moving the package, her remains were discovered and examined by physicians who confirmed Wainwright’s guilt with twelve points of similarity. He was hanged.

Limelight has a not so obvious association with quicklime; “in the limelight” has entered the lexicon so profusely that its original meaning has long been forgotten. In the early 19th Century, Goldsworthy Gurney discovered that quicklime gave off an intense light when heated to elevated temperatures with an oxygen-hydrogen blowtorch. The actual mechanism of this effect is still unsettled, and is attributed to either incandescence by virtue of its temperature or candoluminescence by virtue of selected thermal emissions. The “lime light” was first developed by a Scottish engineer named Thomas Drummond in 1826 for use as an illumination for surveyors, taking advantage of its directionality and focus. Within a decade, limelight was adapted for theatrical productions, premiering at Covent Gardens in 1837. The intensity of the burning lime illumination was ideally suited for spotlights and footlights, which are still called ‘limes’ in Great Britain. Due to the full spectrum natural light effect of the lime-based illumination, actors strove to be “in the limelight” to enhance their personage. Lime lights were supplanted by electric incandescent lights in the early 20th Century. The metaphor remains.

The application of lime to soil for agricultural purposes has historically been a matter of empiricism; the biological role of soil nutrients and the significance of their chemistries having only become manifest in the 19th Century. The primary effect of lime is to control the PH of the soil. Most plants prefer soil that is neutral (PH=7) to slightly acidic (PH=6). However, rain tends to leach out the basic elements such as calcium to make the soil too acidic. Acid rain exacerbates this effect. Because of this, farmers have historically added lime, originally quicklime calcium oxide due to its availability from the mortaring operations, switching to pulverized limestone calcium carbonate as industrialization made this more direct source available. Many farms had rudimentary clamp type lime kilns to satisfy local demand. However, according to Loomis and Wilson in Botany, 4th edition, the interaction of lime and soil is quite complex, and excess lime is as bad as or worse than no lime at all. Lime application affects the availability and toxicity of key soil mineral nutrients, notably phosphorous, iron and manganese. Liming is therefore important for legume crops that require calcium and phosphorous. High levels of lime favor the growth of soil bacteria and deter the growth of soil fungi. It is noteworthy that County Cork in Ireland boasts over 20,000 lime kilns, one for every 80 acres; the fungus Phytophthora infestans was responsible for the Irish Potato Famine of 1845 – 1852.