Historical Overview of Lime Production Methods

Early postcard of the Lime Kilns at Limehouse.
Note Draw Kilns adjacent to the rail line as well as the rail siding.

In the early days of settlement natural cement (gypsum) was mined from beds of limewater shale. The limewater shale occurred naturally inter-bedded with limestone overburdens along the Niagara Escarpment from Queenston Heights to Owen Sound. The lack of good road system necessitated the production of lime wherever the raw materials were easily mined.

Kiln Development

The early industrial production of lime employed batch-operated pot or set kilns. From six the seven days were required to complete the cycle of filling, burning, cooling and emptying. Kiln sizes varied with capacities ranging from six to thirty tons of lime per cycle. The interior or shaft of the pot kiln was generally cup-shaped to cylindrical in form, varying from five to ten feet in diameter and from eight to fifteen feet in depth. Kilns were generally built on the side of knoll and lined with refractory stones (sandstone) laid in mortar. Later refractory bricks were used to line the kilns. The kiln was carefully piled full of limestone pieces with a maximum dimension of twelve to fifteen inched and a minimum size of three inches cubed. Sometimes the logs and limestones were covered with clay, leaving openings at the top and bottom for draft.

The mixing of fuel and raw material in the set kilns resulted in impurities from the fuel being introduced into the finished lime product, which had to be sieved out of the cooked limestone.

In the 1860's and 1870's vertical draw kilns with capacities of 6 to 18 tons per day were first used. Because of their greater economy of operation, the vertical draw kilns quickly superseded the pot kilns, thus bringing about a startling reduction in the number of lime manufacturing establishments in the Province.

The draw kiln was patterned on stack or blast furnaces used to reduce iron ore and other metals. Draw kilns were constructed of non-refractory and refractory-stone; non-refractory (limestone) stones were employed as casings, walls, pillars or other supporting parts of the structure and refractory stone (sandstone) was used to line areas in immediate contact with the fuel and flame such as the lining of the kiln, the fireplace arches and flues. Provision for thermal expansion of dissimilar materials (Limestone, Sandstone, Lime Mortar) necessitated the use of steel and wood tie rods and uprights (buck staves) to keep the masonry kilns from collapsing.

General cross sections of a Draw Kiln.

Draw kilns operate under the principal of gravity. Limestone is fed into the top of the kiln and the cooked stone removed from the bed of the kiln. Fireplaces were located at the sides of the kiln where fuel was burnt to cook limestone. The great advantage of the draw kiln was that it could be operated on a continual basis and was more efficient from the standpoint of fuel economy. Later improvement to the draw kiln, which improved their fuel economy and allowed greater control of the firing temperatures was the introduction of water powered bellows.

The profile of the combustion chamber was generally circular to oval in plan and the vertical section suited to the angle of repose of the charge, in this case the limestone. The raw material was broken up to a minimum size prior to being fed into the kiln. The angle was also dependent upon the specific physical and chemical properties of the limestone charge.

Rotary and vertical gas kilns were introduced starting in 1900.0 Vertical kilns with centre gas burners and an intricate system of draft control and hot gas circulation with capacities of 75 tons of lime per day superceded draw kilns. The North American Chemical Mining and Manufacturing Company, of Shallow lake Ontario, purchased the first rotary kiln in Canada in 1888. The kiln due to its small size could not reach a suitable temperature and the kiln was abandoned. The next use of a rotary kiln was in 1899 by the Beaver Portland Cement Company at Marlbank, Ontario.

The process of lime burning was carried out by kilnsman who were experienced in the reduction of limestone. The quality and quantity of lime produced were dependent upon an number of factors such as the proper temperature, type of wood and the state of the atmosphere. An experienced kilnsman was required to monitor the variables to reduce the amount of "dead burnt" lime. Dead burnt lime will not slake with water and is due to two causes: Insufficient burning and in the case of hydraulic limes from burning at too high a temperature.

View of the Mill Pond looking north from the Toronto Interuban Rail line.
Note the sawmill and to the left the slaking shed and Draw Kiln.
Also note the masonry pier to right of the photograph adjacent to the Mill Pond

Lime Slaking

The slaking of the burnt limestone was the next step in the production of lime. Cooked limestone chunks were removed from kiln and transported to the slaking shed. From a chemical perspective lime slaking involves the addition of water to the cooked limestone. The addition of was forms a hydrate of lime and involves the addition of 1 part water to 1 part lime.

Slaking is the most important part in the production of coarse stuff and lime putty. Unless the slaking is carefully done the resultant materials are liable to blister. The blistering is due to small particles of lime, which if not reduced to a hydrate of lime will blister when employed in plasterwork.

There are three methods of slaking lump lime. The first method is immersion in water, which produces a 'rich or fat' lime. The second method is to sprinkle with water. The third method is to allow the lime to sit and take up water from the atmosphere.

The first method, immersion, produces lime putty or paste, which gains in strength, the longer the lime, is allowed to sit. The ancient Roman law prohibited the use of lime putty unless it had been allowed to set for three years. Coarse stuff (building lime) was generally slaked by immersion. The lime would be run into a pit, the sides of which were formed of boards or masonry. When the lime was slaked it was lifted out by a pail and poured through a coarse sieve.

The second method involved the breaking of the lump lime (cooked limestone) into small pieces, placed in layers of about six inches thick, and uniformly sprinkled with water and covered quickly with sand. It was left in this state for 24 hours before being turned over and passed though a riddle (coarse sieve approximately ¼" grid). The layer of sand serves to retain the heat and allows the slaking to proceed slowly throughout the mass. No water was to be added or the mass disturbed once slaking had begun. Lumps that were not slaked were discarded or placed in the middle of the next heap to be slaked. Generally, lumps, which do not slake are impurities found in the raw limestone or due to under or over firing of the stone in the kiln. A certain percentage of any draws of cooked limestone would include under and over fired lumps as well as impurities. It was the skill of the kilnsman in firing the batches, which would determine the amount of waste. Coarse stuff was run into a pit, the sides of which were made of boards, bricks or sand, left to sit and put through a sieve. The quantity of water added was carefully monitored for if an excess of water is added a useless paste is formed. If insufficient water is added a dangerous powdering lime was produced. Slaking by the second method was frequently done in an incomplete manner and portions of the lime continue to slake long after the mortar had been used.

The third method of allowing the lime to slake by taking up moisture from the air produced an inconsistently slacked lime, however, the slowness of the process as well as the requirement to shelter the lime for an extended period made the process unsuitable for industrial production of slaked lime.

Later mortar mills were employed to reduce or grind slaked and raw lime to improve the consistency of the finished lime product. Grinding served to thoroughly mix the materials, therefore reducing the possibility of blistering. Grinding was generally reserved for the production of lime for the plaster industry.

Reprinted with permission from John McDonald’s “Halton Sketches Revisited”.

For an article on the Kilns (reprinted by permission) from the Niagara Escarpment Commission's Cuesta magazine, click here (large Adobe Acrobat document, 2.8 meg.)

For more on the history of the historic village of Limehouse, visit the Esquesing Historical Society's website, and the article on Limehouse published in the local Tackaberry Times. Also see the recent article on the Kilns in The Country Connection Magazine.

For two newspaer articles from the mid-1960's on consevation efforst concerning the kilns, click here (Adobe Acrobat document 0.5 meg.)