The cremation occurs in a crematory that is housed within a crematorium and comprises one or more furnaces. A cremator is an industrial furnace that is able to generate temperatures of 870–980 °C (1,600–1,800 °F) to ensure disintegration of the corpse. A crematorium may be part of a chapel or a funeral home or may be an independent facility or a service offered by a cemetery.
Modern cremator fuels include oil, natural gas, propane, and, in some areas like Hong Kong, coal gas. However, coal and coke were used until the early 1960s.
Modern cremators have automatically monitor their interior to tell when the cremation process is complete. The time required for cremation varies from body to body, and, in modern furnaces, the process may be as fast as one hour per 50 kg (100 lb) of body weight.
A cremator is not designed to cremate more than one human body at a time; cremation of multiple bodies is generally illegal in the United States and many other countries, though exceptions may be made for (for example) still-born twins, or a baby and mother who died during childbirth.
The chamber where the body is placed is called a retort and is lined with heat-resistant refractory bricks. Refractory bricks are designed in several layers. The outermost layer is usually simply an insulation material, e.g., mineral wool. Inside is typically a layer of insulation brick, mostly calcium silicate in nature. Heavy duty cremators are usually designed with two layers of fire bricks inside the insulation layer. The layer of fire bricks in contact with the combustion process protects the outer layer and must be replaced from time to time. The coffin or container is inserted (charged) into the retort as quickly as possible to avoid heat loss through the top door. The container may be mounted on a charger (motorised trolley) that can quickly insert it, or on a fixed or movable hopper that allows the container to slide into the cremator.
Some crematoria allow relatives to view the charging. This is sometimes done for religious reasons, such as in traditional Hindu and Jain funerals.
In some countries including the United States, there is increasing use of the alkaline hydrolysis process, trademarked as Resomation, which involves the use of lye heated with the body at high pressure, allowing the body to be broken down into its chemical compounds. The process is described by its inventors as more ecologically favorable than other forms of cremation.
In the United States federal law does not dictate any container requirements for cremation. Certain states however may require an opaque or non-transparent container of all cremations. This can be a simple corrugated-cardboard box or a wooden casket (coffin). Most casket manufacturers provide lines of caskets that are specially built for cremation. Another option is a cardboard box that fits inside a wooden shell, which is designed to look like a traditional casket. After the funeral service, the box is removed from the shell before cremation, permitting the shell to be re-used. Funeral homes may also offer rental caskets, which are traditional caskets used only during the services, after which the bodies are transferred to other containers for cremation. Rental caskets are sometimes designed with removable beds and liners, which are replaced after each use.
In the United Kingdom, the body is not removed from the coffin and is not placed into a container as described above. The body is cremated with the coffin, which is why all British coffins that are to be used for cremation must be combustible. The Code of Cremation Practice forbids the opening of the coffin once it has arrived at the crematorium, and rules stipulate that it must be cremated within 72 hours of the funeral service. Therefore, in the United Kingdom, bodies are cremated in the same coffin that they are placed in at the undertaker's, although the regulations allow the use of an approved "cover" during the funeral service. It is recommended that jewellery be removed before the coffin is sealed, for this reason. When cremation is finished, the remains are passed through a magnetic field to remove any metal, which will be interred elsewhere in the crematorium grounds or, increasingly, recycled. The ashes are then given to relatives or loved ones or scattered in the crematorium grounds where facilities exist.
In Germany, the process is mostly similar to that of the United Kingdom. The body is cremated in the coffin. A piece of fire clay with a number on it is used for identifying the remains of the dead body after burning. The remains are then placed in a container called an ash capsule, which generally is put into a cinerary urn.
In Australia, the deceased is cremated in a coffin supplied by the undertaker. Reusable or cardboard coffins are becoming popular, with several manufacturers now supplying them. For low cost, a plain, particle-board coffin (known in the trade as a "chippie") can be used. Handles (if fitted) are plastic and approved for use in a cremator. Coffins vary from natural cardboard and unfinished particle board (covered with a velvet pall if there is a service) to solid timber; most are veneered particle board.
Cremations can be "delivery only", with no preceding chapel service at the crematorium (although a church service may have been held) or preceded by a service in one of the crematorium chapels. Delivery-only allows crematoria to schedule cremations to make best use of the cremators, perhaps by holding the body overnight in a refrigerator, allowing a lower fee to be charged. Delivery-only is sometimes called west chapel service in industry jargon.
Burning and Ashes Collection
The box containing the body is placed in the retort and incinerated at a temperature of 760 to 1150 °C (1400 to 2100 °F). During the cremation process, the greater portion of the body (especially the organs and other soft tissues) is vaporized and oxidized by the intense heat; gases released are discharged through the exhaust system. The process usually takes 90 minutes to two hours, with larger bodies taking longer time.
Jewellery, such as necklaces, wrist-watches and rings, are ordinarily removed before cremation, and returned to the family. Several implanted devices are required to be removed. A pacemakers and other medical devices can cause surprisingly large, dangerous explosions.
Contrary to popular belief, the cremated remains are not ashes in the usual sense. After the incineration is completed, the dry bone fragments are swept out of the retort and pulverised by a machine called a Cremulator — essentially a high-capacity, high-speed blender — to process them into "ashes" or "cremated remains", although pulverisation may also be performed by hand. This leaves the bone with a fine sand like texture and color, able to be scattered without need for mixing with any foreign matter, though the size of the grain varies depending on the Cremulator used. Their weight is approximately 4 pounds (1.8 kg) for adult human females and 6 pounds (2.7 kg) for adult human males. There are various types of Cremulators, including rotating devices, grinders, and older models using heavy metal balls.
The grinding process typically takes about 20 minutes.
In East Asian countries such as Japan, China, or Taiwan, the bones are not pulverised, unless requested beforehand. When not pulverised, the bones are collected by the family and stored as one might do with ashes.
The appearance of cremated remains after grinding is one of the reasons they are called ashes, although a non-technical term sometimes used is "cremains", a portmanteau of "cremated" and "remains". (The Cremation Association of North America prefers that the word "cremains" not be used for referring to "human cremated remains". The reason given is that "cremains" is thought to have less connection with the deceased, whereas a loved one's "cremated remains" has a more identifiable human connection.)
After final grinding, the ashes are placed in a container, which can be anything from a simple cardboard box to a decorative urn. The default container used by most crematoriums, when nothing more expensive has been selected, is usually a hinged, snap-locking plastic box.
An unavoidable consequence of cremation is that a tiny residue of bodily remains is left in the chamber after cremation and mixes with subsequent cremations.
Ash Weight and Composition
Cremated remains are mostly dry calcium phosphates with some minor minerals, such as salts of sodium and potassium. Sulfur and most carbon are driven off as oxidized gases during the process, although a relatively small amount of carbon may remain as carbonate.
The ash remaining represents very roughly 3.5% of the body's original mass (2.5% in children). Because the weight of dry bone fragments is so closely connected to skeletal mass, their weight varies greatly from person to person. Because many changes in body composition (such as fat and muscle loss or gain) do not affect the weight of cremated remains, the weight of the remains can be more closely predicted from the person's height and sex (which predicts skeletal weight), than it can be predicted from the person's simple weight.
Ashes of adults can be said to weigh from 4 pounds (1.8 kg) to 6 pounds (2.7 kg), the first figure being roughly the figure for women, and the second, for men.
Not all that remains is bone. There may be melted metal lumps from missed jewellery; casket furniture; dental fillings; and surgical implants, such as hip replacements. Breast implants do not have to be removed before cremation. Large items such as titanium hip replacements (which tarnish but do not melt) or casket hinges are usually removed before processing, as they may damage the processor. (If they are missed at first, they must ultimately be removed before processing is complete, as items such as titanium joint replacements are far too durable to be ground). Implants may be returned to the family, but are more commonly sold as ferrous/non-ferrous scrap metal. After the remains are processed, smaller bits of metal such as tooth fillings, and rings (commonly known as gleanings) are sieved out and may be later interred in common, consecrated ground in a remote area of the cemetery. They may also be sold as precious metal scrap.