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A crematory (also known as a crematorium, cremator or retort) is a machine in which cremation takes place. Crematories are usually found in funeral homes, chapels, cemeteries, or in stand-alone facilities. A facility which houses the actual cremator units is referred to as a crematorium.
Prior to the Industrial Revolution, any cremation which took place was on an outdoor, open pyre. With firewood, and to a lesser extent, coal being the only available fuel options and the low energy efficiency inherent in such a configuration, it is no surprise that cremation enjoyed minimal popularity in densely populated areas up until furnace technology developed during the Industrial Revolution could be applied to cremation to make it more practical in an urbanizing world.
The organized movement to reinstate cremation as a viable method for body disposal began in the 1870s. In 1869 the idea was presented to the Medical International Congress of Florence by Professors Coletti and Castiglioni "in the name of public health and civilization". In 1873 Professor Gorini of Lodi and Professor Brunetti of Padua published reports or practical work they had conducted. A model of Professor Brunetti's cremating apparatus, together with the resulting ashes, was exhibited at the Vienna Exposition in 1873 and attracted great attention, including that of Sir Henry Thompson, 1st Baronet, a surgeon and Physician to the Queen Victoria, who returned home to become the first and chief promoter of cremation in England.
Meanwhile, Sir Charles William Siemens had developed his regenerative furnace in the 1850s. His furnace operated at a high temperature by using regenerative preheating of fuel and air for combustion. In regenerative preheating, the exhaust gases from the furnace are pumped into a chamber containing bricks, where heat is transferred from the gases to the bricks. The flow of the furnace is then reversed so that fuel and air pass through the chamber and are heated by the bricks. Through this method, an open-hearth furnace can reach temperatures high enough to melt steel, and this process made cremation an efficient and practical proposal. Charles's nephew, Carl Friedrich von Siemens perfected the use of this furnace for the incineration of organic material at his factory in Dresden. The radical politician, Sir Charles Wentworth Dilke, took the corpse of his dead wife there to be cremated in 1874. The efficient and cheap process brought about the quick and complete incineration of the body and was a fundamental technical breakthrough that finally made industrial cremation a practical possibility.
Sir Henry Thompson's main reason for supporting cremation was that "it was becoming a necessary sanitary precaution against the propagation of disease among a population daily growing larger in relation to the area it occupied". In addition, he believed, cremation would prevent premature burial, reduce the expense of funerals, spare mourners the necessity of standing exposed to the weather during interment, and urns would be safe from vandalism. On 13 January 1874, some advocates of cremation, including Anthony Trollope, John Everett Millais, George du Maurier, Thomas Spencer Wells, John Tenniel and Shirley Brooks, held a meeting at Thompson's house in London and formally founded the "Cremation Society of Great Britain" which was "organised expressly for the purpose of obtaining and disseminating information on the subject and for adopting the best method of performing the process, as soon as this could be determined, provided that the act was not contrary to Law".
The first duty of the Cremation Society was to ascertain whether cremation could be legally performed in the country, and then to construct a first crematorium. In 1878, a piece of land in Woking on which the crematorium was to be established was bought by Sir Henry Thompson. Professor Gorini was invited to visit Woking and supervise the erection of his cremation apparatus there.
In 1885, the first official cremation in the UK took place in Woking. The deceased was Mrs Jeannette C. Pickersgill, a well-known figure in literary and scientific circles. By the end of the year, the Cremation Society of Great Britain had overseen two more cremations, a total of 3 out of 597,357 deaths in the UK that year. In 1886 ten bodies were cremated at Woking Crematorium. During 1888, in which 28 cremations took place, the Cremation Society planned to provide a chapel, waiting rooms and other amenities there. In 1892 a crematorium opened in Manchester, followed by one in Glasgow in 1895, Liverpool in 1896 and Birmingham Crematorium in 1903.
Crematoria in Europe were built in 1878 in the town of Gotha in Germany and later in Heidelberg in 1891. The first modern crematory in the U.S. was built in 1876 by Francis Julius LeMoyne after hearing about its use in Europe. During that time it was thought that people were getting sick by attending funerals of those recently deceased and that decomposing bodies were leaking into the water systems. LeMoyne built the crematory to cremate bodies in a controlled environment primarily for sanitary reasons. Cremation was used to destroy any organic matter that could cause illness and give families a better way to preserve ashes. Before LeMoyne’s crematory closed in 1901, it had performed 42 cremations.
Some of the various Protestant churches came to accept cremation, with the rationale being, "God can resurrect a bowl of ashes just as conveniently as he can resurrect a bowl of dust." The 1908 Catholic Encyclopedia was critical about these efforts, referring to them as a "sinister movement" and associating them with Freemasonry, although it said that "there is nothing directly opposed to any dogma of the Church in the practice of cremation." In 1963, Pope Paul VI lifted the ban on cremation, and in 1966 allowed Catholic priests to officiate at cremation ceremonies.
In the U.S. only about one crematory per year were built in the late 19th century. As embalming became more widely accepted and used, crematories lost their sanitary edge. To not be left behind, crematories had an idea of making cremation beautiful. They started building crematories with stained-glass windows and marble floors with frescoed walls. By 2008, the cremation rate was 36.2% and was growing about 1 percentage point a year, according to CANA. CANA is the largest organization representing crematories and funeral homes in the U.S. and Canada.
Australia also started to establish modern cremation movements and societies. Australians had their first purpose-built modern crematorium and chapel in the West Terrace Cemetery in the South Australian capital of Adelaide in 1901. This small building, resembling the buildings at Woking, remained largely unchanged from its 19th-century style and was in full operation until the late 1950s. The oldest operating crematorium in Australia is at Rookwood Cemetery, in Sydney. It opened in 1925.
In the Netherlands, the foundation of the Association for Optional Cremation in 1874 ushered in a long debate about the merits and demerits of cremation. Laws against cremation were challenged and invalidated in 1915 (two years after the construction of the first crematorium in the Netherlands), though cremation did not become legally recognised until 1955.
While open outdoor pyres were used in the past and are often still used in many areas of the world today, notably India, most cremation in industrialized nations takes place within enclosed furnaces designed to maximize utilization of the thermal energy consumed while minimizing the emission of smoke and odors.
A human body usually contains a negative caloric value, meaning that energy is required to combust it. This is a result of the high water content; all water must be vaporized which requires a very large amount of thermal energy.
A 68 kg (150 lbs) body which contains 65% water will require 100 MJ of thermal energy before any combustion will take place. 100 MJ is approximately equivalent to 3 m3 (105 ft3) of natural gas, or 3 liters of fuel oil (0.8 US gallons). Additional energy is necessary to make up for the heat capacity ("preheating") of the furnace, fuel burned for emissions control, and heat losses through the insulation and in the flue gases.
As a result, cremators are most often heated by burners fueled by natural gas. LPG (propane/butane) or fuel oil may be used where natural gas is not available. These burners can range in power from 150 kW (0.5 MMBTU/h) to over 400 kW (1.5 MMBTU/h).
Cremators heated by electricity also exist in India, where electric heating elements bring about cremation without the direct application of flame to the body.
Coal, coke, and wood were used in the past, heating the chambers from below (like a cooking pot). This resulted in an indirect heat and prevented mixing of ash from the fuel with ash from the body. The term retort when applied to cremation furnaces originally referred to this design.
There has been interest, mainly in developing nations, to develop a cremator heated by concentrated solar energy. Another new design starting to find use in India, where wood is traditionally used for cremation, is a cremator based around a wood gas fired process. Due to the manner in which the wood gas is produced, such cremators use only a fraction of the required wood; and according to multiple sources, have far less impact on the environment than traditional natural gas or fuel oil processes. 
A typical unit contains a primary and secondary combustion chamber. These chambers are lined with a refractory ceramic brick designed to withstand the high temperatures.
The primary chamber contains the body – one at a time usually contained in some type of combustible casket or container. This chamber has at least one burner to provide the heat which vaporizes the water content of the body and aids in combustion of the organic portion. A large door exists to load the body container. Temperature in the primary chamber is typically between 760 to 1150 °C (1400 to 2100 °F). Higher temperatures speed cremation but consume more energy, generate more oxides of nitrogen, and accelerate spalling of the furnace's refractory lining.
The secondary chamber may be at the rear or above the primary chamber. A secondary burner(s) fires into this chamber, oxidizing any organic material which passes from the primary chamber. This acts as a method of pollution control to eliminate the emission of odors and smoke. The secondary chamber typically operates at a temperature greater than 900 °C (1650 °F).
The flue gases from the secondary chamber are usually vented to the atmosphere through a refractory-lined flue. They are at a very high temperature, and interest in recovering this thermal energy e.g. for space heating of the funeral chapel, or other facilities or for distribution into local district heating networks has arisen in recent years. Such heat recovery efforts have been viewed in both a positive and negative light by the public.
In addition, filtration systems (baghouses) are being applied to crematoria in many countries. Activated carbon adsorption is being considered for mercury abatement (as a result of dental amalgam). Much of this technology is borrowed from the waste incineration industry on a scaled-down basis. With the rise in the use of cremation in Western nations where amalgam has been used liberally in dental restorations, mercury has been a growing concern.
The application of computer control has allowed the cremator to be more automated, in that temperature and oxygen sensors within the unit along with pre-programmed algorithms based upon the weight of the deceased allow the unit to operate with less user intervention. Such computer systems may also streamline recordkeeping requirements for tracking, environmental, and maintenance purposes.
The time to carry out a cremation can vary from 70 minutes to approximately 200 minutes. Cremators used to run on timers (some still do) and one would have to determine the weight of the body therefore calculating how long the body has to be cremated for and set the timers accordingly. Other types of cremators merely have a start and a stop function for the cremation, displayed on the user interface. The end of the cremation must be judged by the operator who in turn stops the cremation process.