Gypsoteca Gypso-take-care!

Preservation of Plaster Casts

In Malta, plaster sculptures and architectural stucco decorations are quite popular. Some of the largest collections on the islands pertain to the Antonio Sciortino and Vincent Apap bequests in the Heritage Malta Collection at MUŻA, as well as the plaster cast copies students produce within the School of Arts, Valletta and the Gypsoteca at the Old University, Valletta.

As in all conservation interventions, the values of such collections must be acknowledged, and this also includes their purpose and function.

The story behind plaster casts and their properties

Most historical plaster casts were not originally made for display and are not meant to look ‘pure’ milk-white. Consequently, respect for the history of these items and collections is also imperative. Thus, for example, those pertaining to the School of Arts are not ‘restored to their former glory’ and various marks on them are actually considered historic.

The earliest evidence on the use of plaster for sculptural purposes was by the Greeks. With the collapse of Rome, the use of plaster casting is assumed to have declined. This is because no references to this art were found until the Renaissance period, when Andrea del Verrocchio1 significantly contributed to the casting in moulds.2 This practice had facilitated the artistic production of works by Verrocchio, as he used natural objects, such as hands, feet and arms, which were required in his studio for reproduction purposes. Following Verrocchio, other artists used casts of dead people’s heads3 in order to execute their works. Since then, plaster casting has been continually used for various other artistic reasons, as well as to create various sculptures.

The material of plaster is essentially produced from the calcination of gypsum (CaSO4.2H2O). The partial dehydration of gypsum results in a hemihydrate (CaSO4.1/2 H2O).4 Upon reaching 107°C, gypsum converts to ‘plaster of Paris’ (2CaSO4.H2O).5 When water is added to this powder, the 1½ parts of water possessed prior to calcination are recombined to form an inert solid substance.6 Plaster of Paris is partially soluble in water especially at room temperature.7

Its porosity and hygroscopic8 content significantly affect the state of preservation of the sculptures. This could also lead to mould growth and the entrapment of dirt9 which makes cleaning difficult. Besides being an inexpensive material which is commercially easily available, it has an added advantage of being cast, shaped or modelled before drying, but it rapidly sets to produce a hard, compact solid.10 However, plaster of Paris also has some setbacks for sculptural purposes, as it lacks elasticity and has to be used quickly since it cures rapidly.11

Manufacturing techniques

Many local plaster sculptures are considered originals as they are plaster sculptures cast from an original clay model. This original model is sculpted or modelled by the artist himself, from which a negative plaster mould is then produced, with its necessary dividing lines due to the undercuts present. Once this negative has set, the original clay is removed and possibly lost. Following this step, a separating agent is applied internally to the mould and fresh plaster is pressed onto the negative. When this latter plaster has set, the negative moulds are removed and the new cast is then further refined by the artist. In some cases, the artist also patinates or colours the plaster to simulate a bronze finish. When added support is required, for instance to stabilize protruding parts, the sculptor might insert wooden or metal armatures. In some cases, the artist may have also used a textile mesh or threads (sometimes animal hair was used in the past) to support and strengthen the plaster.

On a number of plaster casts, small pencil marks or a sequence of dots can be observed, these are done by an artist using a pantograph.12

These must be retained and not cleaned off, as they are original to the artist’s work.

State of preservation

Most sculptures, in the previously mentioned collections, are all entire with no apparent major damages or loss. They are covered in a layer of grime, mainly composed of settled airborne dust which happens over the years. In an overall assessment, most sculptures are not in any urgent need of conservation. They will require a basic intervention of removing, where possible, this accumulation of grime from the surfaces. There are some minor surface stains on some, which may possibly be toned down with cleaning, while cracks and stains may have been caused by the corroding internal supportive structure. Micro surface cracks may be evident but they are not considered as endangering in any way the sculpture’s integrity.

Scratches and abrasives could have been done through past botched conservation procedures possibly by ‘restorers’ who were not formally trained in conservation. These caused abrading to the original surfaces to remove stains or ingrained dust. Stains have been found to be caused by using non-controlled water-based cleaning techniques, such as steam cleaning, surface gels or brush washing. Other improper interventions are done by using strong acidic or alkaline treatments, which penetrate the pores of the sculptures leaving residues. These may not be evident immediately but activated later on when the objects are exposed to humid environments.

Superficial cleaning

It is advised that if cleaning these vulnerable plaster casts is required, a qualified and experienced conservator-restorer is consulted. Regular maintenance can be kept with a soft brush to remove superficial dust. However, at times, apart from soft dusting, the cleaning process may require cleaning tests, gel cleaning, mechanical cleaning i.e. with a surgical blade or cotton swab, consolidation and plaster integration, as well as chromatic integration. Handling should be done wearing gloves as this prevents the direct contact with natural skin oils which may eventually penetrate the plaster and develop into a stain in the future.13 When lifting a plaster sculpture it should be held from the heaviest part, for an even distribution of its weight, otherwise the plaster may develop cracks or breaks completely. The movement of plaster sculptures should be done on carts, cushioned with polyethylene foam,14 to protect them from any possible damages with unnecessary jerks. The suggested packing of these sculptures is to wrap them with acidfree paper, followed with further wrapping in bubble wrap. Great care must be given to protruding edges. These should be packed with thicker cushioning, e.g. foam or bubble wrap, to buffer the impact of possible knocks or scratches. A slight knock to a plaster cast can cause drastic damage to the entire sculpture, thus prudence must be stressed throughout the packing and transportation process. It is also recommended that each item is placed into an appropriate, individually packed crate or box. This container must cater well for the protection of the various protruding parts as well.

Ideally though, it may be suggested that a foam casing for fragile and easily abraded sculptures is used instead of the bubble wrap, to provide a denser protective layer around the sculpture, as tight bubble wrap may cause abrasion every time the sculptures are moved. The foam may easily be carved out to accommodate small sculptures or protrusions with ease. These foam casings can be retained for reuse during the sculpture’s return transportation and even for any other future exhibitions.

Storage and exhibition

Plaster sculptures are recommended to be stored on light-weight shelving in a restricted area within the museum or location, with temperature and relative humidity levels kept as stable as possible. The recommended range of temperature is suggested to be between 13°C and 18°C, while an RH range between 35% and 45% is suitable. Storing plaster sculptures close to windows, for instance, should be avoided, since wetting/drying and/or heating/cooling cycles might lead to exfoliation or delamination.15 For these reasons, it is very important that the storage environment or exhibition area is adequately checked and monitored. In order to avoid biological growth, the storage space should also be kept well ventilated, in addition to low RH levels.16

Dust should not be left to settle on sculptures as this can trap water, which in turn promotes the growth of mould or mildew. Furthermore, to minimize the risk of deterioration from dust accumulation, Tyvec® can be used as a material to cover the objects. Polyethylene plastic sheets, which may be a cheaper remedy, could be used to cover the shelves and thus exclude dust,17 but it must be checked for off-gasses.

It is of paramount importance that plaster sculptures are not left in direct contact with water since plaster is soluble in water and will cause the surface to soften. Storage should be at least at a minimum of six inches from the ground to protect plasters in case of flooding and humidity.18 It is also important to consider the presence of other materials within the object, besides plaster. For instance, sculptures have wooden or metal internal support structures. The variable capacity of different materials to take up and release water from the structure may cause various damaging effects, such as the cracking of plaster. For example, wood absorbs water and expands greater than plaster, causing the plaster to crack. Similarly, metal corrodes and expands causing ‘jacking’ i.e. expanding and pushing out,19 which also results in cracked plaster.

Storage shelves should be cushioned to prevent chipping the plaster sculptures. This could also be evaded by using high-density foams, as for transportation. On the shelves, the sculptures should be distributed spaced out for their easy access, including space on top of the sculptures, for safe handling.20 They must also be secured against theft.

Conclusion

As can be seen from above, prevention is better than cure and all choices, whether for or against conservation-restoration, depend greatly on the ‘values’ of the sculptures and collections. Based on the manufacturing techniques, such works might be the only way that an artist’s authentic artistic qualities and knowledge can be closely observed. Hence, the preservation of these works for present and future generations is essential.

Notes & References

1 Andrea del Verrocchio (c. 1435-1488), an Italian painter, sculptor, and goldsmith. Originally, he was Andrea di Michele di Francesco de’ Cioni who became a master of an important workshop in Florence. Apparently, he became known as Verrocchio after the surname of his master, a goldsmith. A number of important painters were trained at his workshop including the renowned Leonardo da Vinci. Verrochio’s important works were as a sculptor and his last work, the Equestrian statue of Bartolomeo Colleoni in Venice, is generally accepted as a masterpiece

2 Ibid, pp.57, 58.

3 Ibid, p.58.

4 www.artmolds.com/ali/history.plaster.html

[Accessed: August 2020]

5 Rich 1947: p.57.

6 Ibid pp.59, p.60.

7 Rich 1947: p.60.

8 Hygroscopic is the process of absorbing moisture from the air.

9 Conserve O Gram, 8/2, p.2.

10 Rich 1947: p.61.

11 Ibid.

12 A pantograph is an instrument for copying a plan or drawing on a different scale by a system of hinged and jointed rods

13 Conserve O Gram, 8/2: p.4.

14 Ibid.

15 Conserve O Gram, 8/2: pp.2,3.

16 Ibid, p.3.

17 Ibid.

18 Ibid.

19 Ibid, p.2.

20 Ibid, p.3.

Bibliography

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BONE, D., 1998, ‘Plasters of Paris (and Beyond): The Conservation and Display of Plaster Sculpture’, Conservation News, 65, pp.43-46.

DRYMAN-WEISSER, T., 1992, ‘Metal Objects’, in Caring For Your Collections, ed., Whelchel, H. New York: The National Committee to Save America’s Cultural Collections.

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RICH, J.C., 1947, The Materials and Methods of Sculpture, New York: Dover Publications Inc.

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WOLBERS, R. C. AND LITTLE, M. A., 2004, ‘The Surface Revealed: Cleaning of Two Painted Plaster Sculptures’, AIC Objects Speciality Group Postprints, Vol.11, (AIC).

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VELLA, D., 2007, Antonio Scortino in Gozo: on the 60th Anniversary of his Death, Malta: Heritage Malta.

Sharon Sultana