In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: nss@qmj.co.uk marked for Barry’s attention

Q: I am cleaning some marble sculptures, what can I use to help protect them in the future? 

To start with, are you sure the sculptures require cleaning? This is a point that must be carefully considered as it is often contended that the build-up of dirt is an essential part of the aesthetic of a sculpture, particularly one of white marble.

Many sculptures that have been subjected to cleaning in the past have been accused of appearing anaemic, because dirt no longer helps to define the contours and features, often making them appear more two-dimensional.

If the sculptures are not ‘museum pieces’ or historically important and cleaning is to be carried out, it is advisable to determine first what has caused the dirt build-up and whether this has actually led to any damage.

This should then be related to how long the sculpture has been exposed to the environment it is set in.

From this information you should be able to determine if future protection is actually necessary – and it probably is not if the marble has not suffered deterioration.

If there is deterioration it is probably the result of one (or both) of the two most common forms from weathering: disaggregation and dissolution. 

Disaggregation occurs when the crystal grains at the surface slowly become loosened by differential expansion caused by cycles of wetting and drying, heating and cooling, and freezing and thawing. Salt attack may also occur where there is capillary moisture rise from the ground. These effects are mostly superficial but over a long time can lead to serious losses of material.

Dissolution is simply the gradual loss of surface under the action of acidic materials.  The acid may be derived from rainwater, airborne pollutants and even the touch of humans as deposited oils from the skin break down.

If a marble is suffering dissolution there may be little that can be done to prevent futher deterioration as protection systems typically are designed to fill spaces in the stone and cannot prevent attack that is occurring at the surface.

However, a water-repellent may encourage beading of water, which will tend to travel down and off the stone surface more rapidly, potentially reducing the rate of dissolution. 

Sometimes specialist conservators may apply a protective varnish, a common recipe being paraffin wax mixed with cyclohexanone (ketone resin N) and white spirit, which can be removed using white spirit, if necessary.

If a marble is exhibiting signs of disaggregation, this will eventually lead to loss of detail and potentially destroy the essence of the sculpture. The options then are to leave it, protect it or try to address the problem by knitting the stone back together.

Attempting to knit the stone back together will require the application of stabilizers or consolidants such as urethane or acrylic resins.

However, the use of resins may not be possible if there is a requirement for the treatment to be reversible. Resins may also change the appearance of the stone as they are typically non-crystalline and do not re-transmit light in the way that the crystals of the marble do.

A mixture of acrylic and silane has been used successfully to consolidate marble and provide water repellence, but should be applied by a specialist conservator.

In general, then, it is best to leave marble sculptures well alone, particularly if they are indoors, as they are unlikely to suffer tangible damage. If the sculpture is outside and has suffered damage, the best recourse might be simply to bring it indoors. However, as this is often not possible future deterioration will be difficult to avoid. If there are no heritage restrictions water-repellents, notably silane, may be a simple and effective way of reducing future maintenance. For important sculptures the advice of a true specialist in this field should be sought.