No matter how smooth it's appearance, the thing about marble is that it is porous. And those tiny holes mean that dirt can easily become embedded in the stone. This causes discoloration overtime – like in the first photo – and cannot be removed with regular dusting.
One of our recent projects was cleaning a collection of 18th & 19th century marble busts; so in this blog we thought we'd explain a little about how we went about it.
There are various ways of cleaning marble. 'Wet' methods include the use of detergents, spirits, poultices and steam, but because the stone is porous these all carry risks and are invasive. The use of liquid Latex is also effective, however it widely considered overly invasive because the results often make the marble look too clean and new. 'Dry' cleaning is the least invasive method – and as you can see above the results can still be quite remarkable.
'Dry' cleaning involves the use of special conservation foam erasers.
(Think of a pencil eraser and you are basically there.) Essentially dirt is absorbed onto pieces of the rubber, and then they fall off – just like a normal eraser. It is important to go slowly and carefully – particularly with detailed areas – and the erasers need to be shaped to access intricate parts. We also used 'groom stick', which is a putty like substance which lifts and traps particles of dirt. This was very helpful for the detailed parts of the busts.
As you can see below, some of the busts were very high off the ground, so great care had to be taken when cleaning them. The motion of rubbing – no matter how gentle – will cause movement and rocking if done persistently in the same direction, so rotating and changing the angle of approach is very important.
If you'd like to see the results in person, take a trip to the Royal College of Surgeons in London. The Hunterian Museum at the RCS houses an extraordinary collection of anatomical and pathological specimens and objects, and never fails to stimulate and fascinate its visitors.