Skull cleaning tutorial

Cleaning Skulls And Skeletons By Maceration


With maceration, bones are cleaned of tissue by bacteria; it is a rotting process. There are several maceration techniques, of which warm-water maceration at 35 C is the most effective. This temperature is optimal for the bacteria to live, digest and multiply.

When To Use Maceration

With these techniques, all cartilage gets dissolved, and all unfused bones will separate. Therefore, warm-water maceration is not the most ideal technique for complete skeletons of small mammals (up to the size of a rabbit), since putting the many small bones of hands and feet together will be extremely difficult. Also, the cartilaginous ventral ribs will be destroyed, and have to be replaced by artificial ones. A better way to clean these kind of skeletons is with Dermestes beetles, which keep cartilage and the ligaments between the bones intact. All bird skeletons are suitable for maceration, since they have no cartilaginous parts and the number of bones in wings and legs is much less than in a mammal or reptile. The building plan is very uniform, so if you know one skeleton, you’ll know most of them. Mammal skulls do not have many loose parts, except from the teeth that will fall out often, so most of them can be macerated very well.

Skulls less suitable for maceration are:

  • Small rodents. They will fall apart in many tiny and fragile parts.
  • Bats. The teeth are very small are very difficult to put back
  • Quails and other small fowl species. Fowl skulls always fall apart, and the smallest ones are difficult to reconstruct.
  • Reptiles and amphibians. These fall apart in many small elements. Only the largest species, like goannas, iguanas, boas and pythons have big enough parts.
  • Small fish. Fish always fall apart but if the fish is not too small and the cranium is wrapped with  a string to prevent if from falling apart, fish can be macerated and put back together.

All these can better be cleaned with Dermestes beetles. This handout will mainly concentrate on bird skulls and skeletons.


A closed container is needed, like a plastic bucket with lid, a glass jar or a metal can. Everything will do, as long it is non-corrodible. It is necessary to put a lid on it, to avoid too much vaporizing of the water, and heath loss, and to minimize the smell. It is important to know that maceration smells extremely bad. If you can’t place the container in a fume board, it’s unwise to use this technique indoors.

The easiest way to keep the temperature at the desired 35 C is to put the container in an incubator. If this is not available, an aquarium thermostat will do. If such a thermostat is used, the container should be wrapped in isolation wool or alufoil to avoid heath loss, and make it easier for the thermostat to keep the temperature at the right level.  However, the containee=r should not be closed airtight. Lack of oxygen will slow down the maceration process considerably.

Furthermore, a scalpel, a pair of tweezers, some small mesh fabric , and string is needed.


First of all, all skin that is covered with feathers or hair has to be removed. Hair and feathers are not broken down during maceration, and it will be a great nuisance when all other tissue is digested. While skinning, one has to be careful not to cut off the wing tips and thumbs. While skinning the head, cutting of protruding eyebrow bones, cranium parts around the ear openings and skull base, and jugulars must be avoided. The eyes are removed as well. They contain a ring made of a lot of small bone parts, but this cannot be kept intact with maceration (it can with Dermestes). Therefore, it’s better to remove them in advance, so only the desired bones remain. In a complete skeleton, the hyoid bones (the tongue) should be preserved, so the tongue will be macerated as well.

The beak sheat is left on the head, and should not be cut. Bird feet, covered with horny scales, need not to be skinned. This skin can be removed at the same time the nails are taken off (see below).

To keep the vertebrae in the right sequence, a piece of metal wire can be put through the medulla channel. Sorting out the vertebrae is not that difficult, but this saves some work. The skeleton is separated in the following parts: head plus tongue, body (eventually neck), wings, legs and tail.

All parts that are put in the maceration container are packed in pieces of net curtain. This will allow liquefied tissue to leak out, so in the end, only clean bones remain inside the package. Instead of net curtain, panty hose, cheese clothe or micromesh nylon bags can also be used, especially if the smallest parts may fall through the net curtain pores. A whole skeleton is divided over three parcels: Left wing and leg, Right wing and leg, and body plus neck. Head and tail are added to one of these three groups, preferably with the leg-wing pairs. Eventually, one leg-wing pair can be combined with the body. If one is not familiar with the toe bones, the toes of one foot can be packed separately as well.

The bags are closed with a piece of string, long enough to hang part of it outside the container, so they can be taken out easily.

The Maceration Process

The container is filled with warm water, not up to the rim. It should not be warmer than 50 C, to avoid killing the bacteria. Eventually, some washing powder with enzymes (like Biotex) can be added. The enzymes will help the maceration process, and the soap reduces the bad smell considerably. However, no soap must be added before the beak sheats and nails are taken off, these will be damaged by the soap!

The temperature of the incubator or the thermostat is set at 35 C. If a thermostat is used, it better not rest at the bottom of the container, it works best when it can hang freely in the water. Also, the container should be wrapped with isolating material, to make it easier for the thermostat to maintain the desired temperature. This should not be done in an incubator, where the heat comes from outside the container.

After two days, the water is changed. Some of the old water is added to the new bath, to maintain the bacteria. If a big, greasy mammal is macerated, it is recommendable to remove as much meat and fat as possible after two days, to avoid too much fat accumulation in the skull.

It differs per animal and per occasion how long it will take to digest all tissue. Usually, one can check after about 5 days. If there is still tissue left, just put them back until it is all gone. Normally, the process won’t take longer than 14 days. If the process is still not finished by that time, it may be halted (see below).

When the bones are clean, they are taken out of the container, rinsed in clean running water and laid on absorbing paper to dry at room temperature. They should not be dried in an oven or in bright sunlight, to avoid cracking.

Beak Sheat And Nails

Most skull collectors prefer to preserve the horny beak sheet, and in complete skeletons the nails. If the bird was fresh, these parts can be removed after 2-3 days in the water bath, by gentle pulling. If the sheats are very delicate, like those of estrild finches and long-billed waders, the beak sheat should be made loose at the bill base with a scalpel or needle, to keep the beak sheat in one piece. In dried out birds, the beak sheat often does not come of before the horn starts decomposing. In this case, the sheat or nail can be loosened with a needle or a pointed scalpel.

The horny parts can be stored dry, frozen or in 9% formalin solution. The latter method is especially recommended for birds with colorful bills. The formalin will prevent fading later.

If The Process Stops

Sometimes, the degradation of tissue stops. This may be due to development of the wrong bacteria. The remaining tissue can be removed by warming up the water bath to 80 C, with soap added, for 24 hours. This temperature is under normal circumstances not recommended, because fragile bones get deformed easily.

Another possibility is to bring the water to cooking point, let it simmer (not boil) for about an hour, and put the skulls back into a fresh 35 C water bath. The maceration process should resume this way.


Removing grease from the bones is very important. Greasy bones look dirty, attract more dust and don’t bleach very well. There are several degreasing agents, all with pros and contras:

  • Benzine; an easy to obtain, rather cheap and good degreaser. Volatile, so a closed jar is needed. Several plastics are benzine resistant.
  • Acetone, a very good degreaser, but rather expensive. Highly volatile, requires a glass or metal jar, because it dissolves most plastics. Can be recycled by freezing out the grease at –18 C.
  • Trychloroethylene, said to be the best degreaser. Similar features as acetone, but toxic and carcinogenic. Not recommended outside a laboratory.
  • Ammonia; a fairly good and degreaser, recommended for big skulls. Used undiluted or not more diluted than 50%. Somewhat volatile with very irritating vapors. Can be used only once.
  • Hot soap; Very cheap, water with dishwashing soap, warmed to 80 C, which must never boil. Can only be used in an incubator or with a powerful thermostat. Only for big, sturdy skulls, not for fragile ones.
  • Drilling holes in the short ends of hollow bones helps the degreasing process a lot.

Sometimes, and with mammals often, a white, waxy stuff remains on the skull. This is called adiposere, concentrated grease. It I hard to remove, but after degreasing, if often has lost it’s stickyness, and can be scraped or brushed away easily.

The length of the degreasing period depends on how greasy the bones are; a minimum of 14 days is recommendable.


The best bleaching agent is hydrogen peroxide (H2O2). In a concentration of 4%, it bleaches most bones in 24 hours, or in a few days. It irritates the skin, so gloves should be worn. During the bleaching process, oxygen gas escapes, so the bleaching container must never be closed airtight!  Keep the peroxide out of the sun to avoid breakdown.

A 4% peroxide solution can be used 2-3 times, until it no longer foams in contact with organic matter. The bleaching effect can be sped up and enhanced by warming the solution, but this causes rapid inactivation of the peroxide as well.

After bleaching, the bones are dried in open air on absorbing paper.

Articulation Of Bird Skulls

Figure 1. Major skullparts described in tutorial

The articulation of complete skeletons is too complex to include in this manual. Skulls of most adult bird do not fall apart during maceration. Only a few parts, which articulate with joints with the rest of the skull, will separate: the lower jaw, the quadrates (articulation bones between cranium and lower jaw), and the pterygoids (rod shaped little bones between quadrate and palate) (see also figure 1). Furthermore, in many birds the eyebrows and the vomer also separates (see below). If a bird is not fully matured yet, other elements that will fuse later in life may separate during maceration as well. A bird skull is made up of maximally 36 skull bones. On the other hand, in old birds, some parts may fuse that normally come loose. In case a part is missing, one should check whether it is still on the skull to start with.

The best glues for rearticulation of bones are white wood glue and hot glue. Both dry up transparent, and the glue does not dry out so much that it may crack.

Articulation starts with putting the quadrates back in place. Then, before the first glue connections are fully hardened, the pterygoids are put in place. For the inexperienced, this can be rather difficult. The eyebrows and vomer are the last bones that are glued in.

If the beak sheat is stored dry, it is made flexible again by soaking in warm water. Those stored in formalin are transferred to clean water.  In both cases, the sheat is wet when it is put on. Some glue is put inside; the sheat is slid back on the bill, where horn and glue will dry simultaneously, in the right form.

It is important to put the beak sheat back on before the other glued points have fully hardened. If the orientation of bill, quadrates and pterigoids is not exactly right, the bill won’t close. If the glue is still somewhat soft, the elements can be forced in the right orientation. The right orientation is maintained during the drying process with tape or wire. If there is a lot of tension in the skull, some extra glue can be added to the connection points. Whether the lower jaw is glued on the skull or not is a matter of opinion.

When the skull is dry, the outer layers of the horny beak sheat will often flake off, giving the bill a dull, ragged appearance. These outer layers can be missed, and scraped (not cut) off carefully with a scalpel. Then, the dust is buffed off, leaving a smooth, shiny bill. The bill can be kept shiny with some Vaseline or with clear lacquer.

Adult birds that fall apart

In most birds, only the skulls of juveniles fall apart during maceration in the separate skull bones, but in a few families, the skulls of adults fall apart as well, except sometimes in very old individuals. These are:

  • Struthioniformes (Ostrich)
  • Rheiformes (Rheas)
  • Casuariiformes (Emus and Cassowaries)
  • Sphenisciformes (Penguins)
  • Galliformes (Fowls)
  • Psittaciformes (Parrots)
  • Apodiformes (Swifts)

These skulls are a real nuisance (or challenge) to work with, and not advisable to beginners. How many skull bones separate differs per family, and also per individual, but usually more than in other birds.

Birds with loose eyebrows

Whether a bird has loose eyebrows that come off during maceration differs per order or family. Often, these parts are very small and not initially recognized as part of the skull. Therefore it is good to know in what groups one should be alert for these parts. Unfortunately, the author has experience with only 96 of the 172 bird families, and 24 of the 27 orders. Whether the Apterigiformes (kiwis), Tinamiformes (tinamous) and Caprimulgiformes (nightjars) have loose eyebrows could not be determined. The orders that do have loose eyebrows in adult birds are:

  • Struthioniformes (Ostrich)
  • Rheiformes (Rheas)
  • Casuariiformes (Emus and Cassowaries)
  • Sphenisciformes (Penguins)
  • Podicipediformes (Grebes)
  • Ciconiiformes (Storks, Herons, Ibises, Spoonbills and Flamingos)
  • Falconiformes (Birds of prey, all have two-part eyebrows, except osprey, vultures and falcons, that have one-part eyebrows)
  • Galliformes (fowls)
  • Gruiformes (Cranes, Rails and Bustards)
  • Cuculiformes (Cuckoos and Turacos)
  • Trogoniformes (Trogons)
  • Coraciiformes (Rollers, Bee-eaters, and Kingfishers, not the Hornbills)

Birds with a loose vomer

The vomer is a small, forked bone situated in the middle of the upper beak. In some groups of birds, it may separate during maceration. These groups are:

  • Struthioniformes (Ostrich)
  • Rheiformes (Rheas)
  • Casuariiformes (Emus and Cassowaries)
  • Sphenisciformes (Penguins)
  • Gaviiformes (Loons)
  • Podicipediformes (Grebes)
  • Falconiformes (Birds of prey)
  • Charadriiformes (Waders, Gulls, Terns, Skuas and Auks)

Ducks and other soft billed birds

The bill of ducks, swans and geese are not completely covered with horn, only the tip (the nail) is, the rest is a leathery kind of skin. Flamingos and spoonbills have similar bills. This beak skin will be destroyed during maceration. If you want to keep the whole bill covered, each head is put in a separate jar, which is filled with water just up to the bill base. The jars are put in a 35 C incubator, where they are macerated, except the bill. Because the water level has to be kept constant, and to prevent floating, the jar has to be closed airtight, and floating of the head has to be prevented. The water has to be changed every other day to avoid oxygen depletion. When all tissue is degraded, the heads are taken out, dried, degreased and bleached and again, the bills are not submerged.

A bit of luck is needed to get a good result with this method, the bill skins get damaged often anyway. Also, the peroxide will creep up, causing discoloration of the basal part of the bill. Furthermore, the skull will smell bad for a long time. When skull and bill are fully dried, the bill can be protected with clear lacquer.