Reproducing Our Ancestors

The University Museum's Casting Program

By: Alan Mann and Janet Monge

Originally Published in 1987

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A hundred and thirty-one years ago, quarry workers in the Neander Valley (“Tal” in German) came across the mineralized remains of a brutish-looking, primitive creature (Fig. I). For several years, there was considerable debate as to the nature of this animal. Initially identified as a bear by the quarry owner, the fossils were later considered to be the remains of a modem human. Strange characteristics of the remains, such as the long, low form of the skull, the heavy brow, and the short bowed leg bones were explained as the result of disease or foreign origins, so that the Neandertal bones became: a person who had been washed away by Noah’s flood; an ancient Celt or German; or a Mongolian Cossack who deserted from the Russian cavalry. But by 1864, following the publication of Darwin’s The Origin of Species, the Neandertal bones took their place as the first human fossils to be recognized—the initial testament to the evolutionary development of our species.

Since that time, many additional bones and teeth (not to mention tools of stone and bone and even artistic works of great power) have furnished more documentation of our biological past. The most primitive of the fossil remains are 5 million years old, and illustrate our evolution from small-brained, ape-like creatures.

Though many of these fossils are broken and incomplete, and have been distorted through their long interment in the ground, they represent our only direct links with our ancestors of 100, 1,000, 10,000 and even 1 million generations ago. Studying them reveals the pathways followed by our line during its long evolutionary journey, and provides precious insights into the nature and origins of modern human biology and behavior.

Recently, the Casting Program of The University Museum has looked into ways to preserve these fossil specimens. In most cases, the original bones remain in the countries where they were found. Using modern plastics and synthetic rubber technology, we have been developing techniques and procedures that can be used to produce replicas that are as accurate as is possible. The need for such a project becomes clear when we consider how scientists study fossil bones and the fate of many already discovered specimens.

The Value of Fossil Casts

The use of casts of fossil bones, fabricated from such diverse materials as paper, wood, plaster, rubber, and the variety of plastics, is a long-standing tradition in all fields of paleontology, and represents a crucial link in the rela­tionship between scholars and the evidence they study. Casts preserve the external form of the fossil, and they thus represent a permanent duplicate record of the shape of fossil bones. They are routinely used in place of original fossils for re­search, since they enable scientists to study and compare the remains of animals that have been discovered thousands of miles apart, and may be stored on different continents. Virtually every paleontological mu­seum and academic department spends considerable time in the pro­curement of quality casts for both research and instructional purposes.

The need for accurate replicas of fossil bones is not just a matter of convenience for students and re­searchers, however. In the late au­tumn of 1941, just before the attack on Pearl Harbor that would bring the United States into the Pacific War, the staff of the Peking (now Beijing) Union Medical College made the decision to pack up the unique and precious fossil human bones excavated at the site of Chou­koutien (now Zhoukoudian) and ship them off to the United States for safe keeping. Sometime after these bones—best known as the Peking Man fossils—left Beijing in the company of the U.S. Marine contingent of the embassy, the fossil collection was lost or destroyed. In the years since, many individuals have sought information regarding their whereabouts, and enormous rewards have been posted for their return. Nevertheless, the fossil bones and all they represent as documents of our evolutionary history have remained lost.

Fortunately, during the time when they were studied in China, quality plaster casts of almost all of the Zhoukoudian bones were made and distributed to major museums a­round the world (Fig. 2). These casts preserve a remarkable amount of detail, and in many cases, meas­urements taken from them show no significant difference from meas­urements recorded on the original fossils. This represents a remarkable achievement considering the level of molding and casting technology in the 1930s, and the (by today’s standards) primitive molding media. Although no cast is an ideal substitute for the original fossil, in this case, the casts represent the only record of these specimens, and provide a reasonable alternative for the mis­sing originals.

The loss to science of the Peking Man fossils does not represent the only such disaster to befall the bony relics of our evolution. During the final stages of World War II, the museum in Brno, Czechoslovakia, was heavily damaged. In the resul­ting fire, much of a collection of 29 skeletons recovered from the unique Upper Paleolithic sites of Predmosti was destroyed. Unfortunately, only two Predmosti specimens, both skulls, were molded and cast prior to their destruction. These casts have become the “types” considered representative of the original sample of bones. An examination of photo­graphs of other specimens pictured in a 1934 monograph, however, shows that the collection was a variable one. We may speculate as to how different our views of the origin of modern humanity might be today if the Predmosti samples (or ample casts) had been preserved into the period after World War II, when our appreciation of the evolu­tionary significance of population variation began to develop.

Inclusion of fossils in a museum collection does not guarantee their safety. Ancient bones are very fre­quently fragile: some are only par­tially mineralized or fossilized, oth­ers are not mineralized at all. Often, they are held together by tooth­picks and bits of wire. With the interest in human origins expanding each year, and the number of profes­sional scholars involved in active research steadily increasing, these fossils are being handled, examined, photographed, measured, and bro­ken with greater frequency. Many researchers have had the experience of examining a fossil they had first looked at several years before, only to find that, due to handling or an accident, the fossil specimen is now different in appearance.

This is a sensitive area in paleoan­thropology, but one case that can be discussed is that of the famous South African specimen Sterkfontein Skull 5 (Sts 5), the most complete example known of the skull of Australopithe­cus africanus. In 1950, Sts 5 or “Mrs. Ples” was accidentally dropped. There was considerable damage to the top of the skull, with serious loss of bone. The reconstructed skull differs from the original in form, but the exact nature of the difference has been the subject of much debate.

Less dramatic is the damage caused by simple wear and tear. Many famous and important fossils have been measured so often that there are actually grooves or pits in the places where measurement land­marks are located. Hominid fossils dating to the Later Pleistocene, sometimes not completely mineral­ized, are literally worn away as they are handled. In the labs and offices where such fossils are studied, nu­merous tiny flakes of bone are often found lying around the work tables and storage drawers. Though this is not the situation in Kenya, concern for the fossils has led Richard Leakey to propose a five-year moratorium on the examination of fossils housed at the Kenya National Museum in Nairobi. There is serious concern among some scholars as to what will remain of all hominid fossils a hun­dred years from now.

Molding and Casting of Fossil Bones: A Brief History

During the 19th century a large array of molding and casting media was em- played in making replicas of fossil bones, but plaster was by far the most widely used. For example, in 1900 the Carnegie Museum spent a considerable sum to mold a com­plete dinosaur skeleton, whose plas­ter-of-Paris casts were later distribu­ted to many other museums around the world.

In the 1920s and 1930s, F.O. Bar­low, a leading mold and cast maker in Europe, fabricated more than 600 plaster-of-Paris molds of homi­nid fossils. In order to ease the removal of plaster from an often fragile fossil specimen, these molds were made of many pieces, waxed and keyed to each other. A complete mold of a skull, for example, could easily be composed of more than 100 plaster pieces, each replicating a small part of the whole fossil; fitted precisely together, they form what can only be described as a large three-dimensional jigsaw puz­zle. Casts produced from Barlow’s molds have a surprising amount of detail. They appear to be dimen­sionally accurate replicas, but many cannot be adequately assessed be­cause in the meantime, the original fossil has been modified, reconstruc­ted, or even damaged. About 500. Barlow-produced molds of fossil hominid bones have survived, and are now in The University Museum Casting Program collection (Fig. 3).

One of the outstanding features of the Barlow collection is the pres­ence of many molds of fossil human remains found in the early part of this century. While it is sometimes unclear just what “stage” in the fossil’s history the Barlow mold rep­resents, those made relatively soon after the fossils were found should preserve the bones in a condition close to that at the time of excava­tion.

Other molds, made of fossils after they had been reconstructed, are of historical interest, providing an important glimpse into the ways by which a fossil specimen may change as a result of modern human interpretation. Many of these specimens have been “re-reconstructed” since Barlow’s time, to conform to other expectations and different interpre­tations of the human fossil record.

Since 1952, when the Barlow col­lection came to The University Mu­seum, the molds have been used to produce casts. For practical reasons, latex rubber molds were made from the plaster-of-Paris originals. From these, thousands of fossil casts (in­cluding the original Neandertal finds, those from Cro-Magnon, Pe­king Man, and the Taung child) were made and distributed to hun­dreds of museums, colleges, and universities. Because for many years it was the major source for casts, it is not an overstatement to say that The University Museum provided several generations of scholars with their only replicas of hominid fossils.

In the 1960s, the Wenner-Gren Foundation for Anthropological Research sponsored research which led to the development of new molding and casting techniques and the introduction of more modern materials. Lita Osmundsen, Director of Research for Wenner-Gren, saw the need for more accurate and detailed casts of fossil hominid bones. The Foundation enlisted the services of plastics expert David Gilbert, who spent a number of years developing workable proce­dures for employing newly intro­duced synthetic silicone molding rubbers, and a variety of plastic casting resins.

From 1962 to 1976 the Wenner­Gren Foundation sent mold makers all over the world. Using techniques developed by Gilbert, they were able to produce over 900 molds of the most famous human and primate fossils, including Zinjanthropus or “Nut Cracker Man,” Mrs. Pies and the Taung child from South Africa, and Kabwe Man. During this time the Wenner-Gren Foundation pro­duced nearly 16,000 high-quality plastic casts of fossil remains, and distributed them to scientists and research and teaching institutions throughout the world (Fig. 4).

A major objective of the Wenner­Gren program was the development of methods to insure that the accuracy and details captured by the sili­cone rubber molds were maintained over time. This crucial need was met by the concept of the “pattern,” a device that stores the form of a fossil, and permits many generations of highly accurate casts to be produced (see below). Such patterns form the core of a new project intended to safeguard for posterity the fossil bones documenting human and primate evolution, in replica form.

The 900 Wenner-Gren molds and patterns and the 500 Barlow plaster-of -Paris molds form the nucleus of this repository of bone replicas, established by The University Muse­um with the support of the National Science Foundation and the Wen­ner-Gren Foundation. It is the first and only repository of this type in the world. Using the techniques developed by the Wenner-Gren Foundation, another 200 silicone molds have been added to the col­lection by University Museum mold makers, and the Casting Program is dedicated to adding new molds in the future. The National Science Foundation is funding the inroduc­tion of patterns, and the long-term curation of both molds and patterns.

The Casting Process and the Importance of Patterns

A mold is a negative impression of an object (in this case a bone) that is used to make a positive impression. Both patterns (stable epoxy positives or duplicates of a mold) and casts (positives of the bone itself) can be made from these molds. While molding and casting are relatively familiar con­cepts, pattern making is not. Its importance within the casting process derives from the specific tech­nology used to make highly accurate reproductions. A general description of the way in which a fossil cast is made is presented below; technical details are given in the accompany­ing box.

The Mold

A mold is an impression made on the original fossilized bone or tooth (Fig. 5). Since museums safeguard the fossils in their care, making a mold usually requires that the mold maker travel with his or her equip­ment and supplies to the country where these bones are housed and curated. The mold itself is made from a flexible material, today usu­ally silicone rubber; it is backed or strengthened with a solid material, usually a very fine-grained gypsum cement.

Why isn’t a mold, a highly accurate negative impression, stored as the permanent representation of a fossil bone? In order to be safely removed from a delicate bone or tooth, molds must be made of a flexible sub­stance; this is usually a synthetic silicone rubber, but latex rubbers also have been used for this purpose. Over time, these flexible molds lose the property termed “memory,” which allows the mold to return to its original form after it has been distorted—as in pushing or pulling out a rigid cast. Also, the tear strength of the mold is lost over time: when a mold is stretched it should return to its original form; if instead the mold tears where stretch­ing pressure was applied, the mold becomes unusable. When memory and tear strength are lost, the flexible mold ceases to be an accurate record of the original fossil. There is thus the necessity for a stable copy of the mold, a pattern.

The Pattern

Figure 6a-f illustrates the stages involved in pattern production. Each stage serves to guarantee that the pattern will be a most accurate and stable representation of the fossil bone. The specific fossil illustrated here (Fig. 6f) is the upper jaw of a 12-million-year-old ape-like crea­ture. This bone was molded in 1982 in Budapest, Hungary, by University Museum mold makers. With the permission of the excavator, the mold was brought back to the Unit­ed States and became part of The University Museum’s collection. Pattern production took place in 1983.

A pattern is like a cast in that it is a positive impression taken from a mold. A pattern, however, is a posi­tive not just of the hollow in the mold that replicates the fossil, but also of the complete mold section. So a pattern is a positive of a bone or tooth as it was being molded, and the form of the mold is maintained as well as the form of the bone (Fig. 7). Since the surrounding flange, locators, and holes are part of the original mold, they are also incor­porated into the pattern. Because any bone is molded in at least two sections, one mold is represented by at least two patterns.

The material used for the pattern itself is epoxy. As a casting medium, epoxy far surpasses any other in its strength and its ability to remain dimensionally stable. It is not only capable of making an almost perfect replica of the bone form, but is known to preserve microscopic fea­tures of the fossil itself (see below).

The Cast

A cast is produced when the negative impression within a mold is dupli­cated. Why isn’t a cast stored as a permanent record of the mold? First, it is very difficult to produce a perfect cast. This is especially true if the mold is very complicated. When the various sections of the mold are brought together, it is almost impossible to insure that the casting medium is uniformly thick within the mold cavity. Spaces or voids can be created within the cast that represent weak spots; as a result, the cast is likely to distort. Second, the most suitable casting medium, epoxy, causes the mold itself to deteriorate as multiple casts are produced. One silicone rubber mold can produce 10-15 casts, with each consecutive cast resulting in the loss of some detail. Thus not only is it unlikely that the first cast will be a perfect representative of the mold, but each successive casting produces a less accurate cast of the original bone.

The use of patterns insures that thousand of high-quality epoxy casts can be produced (Fig. 8). This is because new generations of molds or “working molds” can be made repeatedly from a single pattern.

Technology of Casting

Making a Mold

A mold of most fossil bones is made of two or more parts. When the design of the mold is being planned at the beginning of the molding process, a division or “part-line” is made at the place on the bone that will provide the most efficient mold, taking into account the safety of the fossil. Since only one side of the specimen will be molded at a time, the side of the fossil to be molded last is encased in plasticine to protect it. The plasticine is then set into a flat plate or flange of plaster (Fig. 6a).

A row of stainless steel rods and plastic locators” with faceted sides are set into the flange of the plaster partline (Fig. 6b). The locators insure that the two halves of the mold will register precisely together, minimiz­ing the possibility of misalignment when a cast is made. Rods are placed on the partline to produce holes which extend through the en­tire mold. Later, when the fossil is removed and a cast is made from the completed mold, screws are inserted into these holes. Tightening the screws permits a controlled amount of pressure to be applied all around the mold during the casting process.

Once the plaster partline is com­pleted and rods and locators are in place, three coats of molding rubber are brushed over the entire surface of the bone and flange. (In order to insure as much detail as possible, no surface coating is applied to the fossil bone.) After the rubber has cured, a hard plaster backing (the “keeper shell”) is poured. The first section of the mold is then complete and the temporary plasticine and plaster flange is broken away from the bone. The second side of the mold is made in an identical manner.

Making a Pattern

Pattern making begins with the preparation of the mold: a wall made of dental wax is constructed around the face to prevent the epoxy and plaster used in making the pat­tern from leaking (Fig. 6b). The mold face is then covered with a 1/4-inch-thick layer of epoxy. Once this epoxy layer begins to cure, a layer of gypsum cement about 1 inch thick is poured over the epoxy to form an outer reinforcement. Thus the pattern is composed of a layer of epoxy, which captures the detail of the surface of the bone, backed with gypsum cement.

Making a New Mold

When a new mold is distorted or torn as a result of pattern making or in the production of casts, a new mold is made from the pattern. In this relatively simple process, sili­cone rubber is brushed on the pat­tern. After curing, the new flexible mold is removed. The locators and steel pins that were designed into the original mold make it possible for these new generations of molds to align exactly when taken from the pattern.

Future Directions

The University Museum pos­sesses the world’s largest col­lection of molds and patterns of hominid fossils, and more speci­mens will, we hope, be added. But it is clear that a single location for the long-term storage of all fossil hominid replicas is not a good idea. The main goal of such repositories—insuring the preservation of the shape of these fossils—is best achieved by having a number of collections located in museums widely distributed throughout the world. Since a mold can produce more than one pattern, it is prudent, especially for important or noted fossils, to consider the fabrication of several patterns, to be stored in different places. Most of the early hominid fossils have been discov­ered in Africa and Asia, and it would be reasonable to establish repositories on these continents, as well as in Europe and Australia. Discussions are currently in progress with representatives of a number of museums and research institutions in various parts of the world for the establishment of other pattern stor­age locales.

It is also important to recognize that pattern manufacture and distri­bution is a major contribution that American scholars and institutions can make to colleagues in the Old World. No fossil hominids have ever been discovered in the New World. The development of storage repositories in different parts of the world can provide an important channel by which American anthro­pology can contribute to the long­term future needs of the field of paleoanthropology.

Further, in the next few years, the production of exact replicas is likely to be extended from fossils to mod­ern human remains. In Australia, for example, there is a possibility that most of the prehistoric and historic hominid bone remains discovered on that continent will be reinterred in the near future under court orders. Proposed legislation in the U.S. may also result in archaeological remains being reburied after study. Without patterns to provide a record of these fossils and archaeological finds, these discoveries will be per­manently lost to the scientific com­munity.

Finally, we are currently investi­gating the ways by which the mold and pattern collection can be sup­plemented to store additional infor­mation about the fossil bones they preserve. Recent research in human evolutionary studies has emphasized the histological and other micro­scopic structural details observable on bone. The technology and mate­rials used in the molding and pat­terning procedures permit the pres­ervation of these structures on pat­ terns. To evaluate this aspect of the patterns, a series of experiments is underway, using recent bones in the physical anthropology collections at The University Museum. If micro­scopic structures can be preserved on patterns, then the research possi­bilities for this collection would be greatly expanded.

While the primary mission of the pattern program is to insure the long-term safety of the fossil record for human evolution, it is also appar­ent that the collection has an enor­mous research potential. Many an­thropologists and other scholars would find the huge number of patterns of fossil bones and teeth in our facility an excellent resource for a wide variety of studies in paleoan­thropology. We plan to make availa­ble to the scientific community a list of the patterns and molds that are in storage and available for study. Our goal is eventually to produce casts from the molds and patterns, so that a scholar who wishes to study a particular fossil will be able to exam­ine a replica virtually identical to the original.

We hope that through ‘these ef­forts, the fossil bones of our ances­tors, having lasted for thousands or millions of years in the ground, will successfully survive additional hun­dreds of years in museum storage.

Cite This Article

Mann, Alan and Monge, Janet. "Reproducing Our Ancestors." Expedition Magazine 29, no. 1 (March, 1987): -. Accessed May 27, 2024. https://www.penn.museum/sites/expedition/reproducing-our-ancestors/


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