This section contains information and advice on how to look after domestic plaster objects. It highlights common problems, tells you what to avoid and provides practical instructions on how to clean and preserve plaster.
First decide if your plaster has historical, aesthetic or sentimental value and whether you wish to pass it on to future generations. If you think it has significant monetary value, consider having it valued, insured and professionally conserved.
Types of plaster
Plaster is quick and easy to use and relatively lightweight. Generally speaking, the plaster used in the decorative arts is plaster of Paris, which is a form of calcium sulphate. It is made by heating gypsum rock to remove the water content of the rock (calcium sulphate hemihydrate) which is then ground into a powder. To make an object in plaster of Paris, this powder is mixed with water and cast in a mould, or occasionally modelled using small quantities of semi-liquid plaster. The plaster 'dries' or sets by chemical reaction.
Once dry, plaster is soft and brittle. As a result plaster objects usually incorporate a support system, such as a metal or wood armature. Hessian or other fibrous materials may be incorporated into the plaster itself or used as a backing. Plaster has been used to make busts, figures, statuettes, sculptors' models, relief panels and architectural mouldings.
Plaster is brittle and many plaster objects, particularly larger ones, are hollow. They are often cast in more than one section which are then joined together during construction, sometimes also with supporting armatures. As a result, the plaster cast varies in thickness. The areas where the separate sections are joined tend to be much thicker, as do the areas attached to the armature. Other areas can be as little as 3mm thick. Plaster objects are very vulnerable to impact damage, from knocks or being dropped. If they are hollow, a small knock can cause a great deal of damage.
Plaster is also very porous, easily absorbing dirt, spills or liquid cleaning materials. It is comparatively soft and easily damaged by abrasives. Artists and sculptors, aware of this problem, often sealed the surface to reduce its porosity.
In terms of caring for your antiques, what matters is relative humidity. Relative humidity measures the amount of moisture that is in the air relative to the maximum amount of moisture that the air could hold at that temperature.
High relative humidity, damp or contact with water can cause several problems. Firstly, the plaster itself is slightly soluble in water. Secondly, the armature that provides support to the structure is vulnerable to high, low or fluctuating relative humidity.
Wooden armatures will expand and contract in response to changes in relative humidity, whilst metal armatures, historically made from iron or steel, may corrode. These corrosion products are larger in volume than the original metal and cause splits, misalignment or breakage of the plaster, which will itself have been damaged by the moist conditions that initiated the corrosion.
The first step in cleaning plaster safely is to identify whether it has been sealed or not. If it has, you also need to identify the material used. Plasters may have been coated or sealed using a wide variety of materials including varnishes, wax, paint and milk, or a mixture of potassium oleate and alum.
Sealed plaster can be easier to clean because it is less porous, but the sealer adds its own potential complications to the cleaning process. Unsealed plaster is difficult to clean without causing damage. Sealed plaster tends to have a slight sheen to the surface, and is less absorbent than unsealed plaster. If in doubt, consult a specialist conservator.
Clean sealed plaster with a swab dampened with water or white spirit. Test a small, unobtrusive test area and, if this is successful, progress in small stages over the remainder of the object. Problem areas to watch out for include flaking plaster and areas of thin coating that might show gaps. Sticky marks may be removed by wiping the surface over with a clean, slightly damp cloth.
Dust can be removed with a soft cloth or brush, though take care not to knock the plaster with the metal ring at the end of the brush (ferrule). Plaster is slightly soluble in water and is vulnerable to damage from domestic cleaning products and abrasives, which will remove the surface layer. Its porous nature means that cleaning should ideally minimise the use of any liquid. Stains that have absorbed deep into plaster will be difficult to remove.
Options for dry cleaning materials include Groomstick, a spongy, natural rubber product that is free from moisture, solvents or chemical additives (supplier: Conservation Resources Ltd); vulcanised rubber smoke sponges or a Mars Staedtler white pencil eraser with minimum pressure (available from stationers). If these methods are not effective on unsealed plaster, consult a conservator about further treatment.
Stripping a plaster bust
For the inexperienced, stripping plaster is difficult to do well. Abrasives are likely to damage the surface whilst paint strippers may penetrate the plaster, causing damage or stains. It is best to consult a specialist, who can offer advice on the best course of action.
However, before consulting a conservator give some thought to the issues involved. For example, why was the bust painted in the first place? A common reason for overpainting is to conceal old stains or disguise old restoration fills. If this is the case, no matter how well the paint is removed, the piece may look no better afterwards.
If peeling paint is the problem, this can be treated by a conservator - stripping is not your only option. Find out all you can about your object. If the paint is an original finish, or if any underlying paint is original, this may add to the historical information and authenticity of the piece. Removing original finishes often reduces both historical and monetary value.