David Francis is a hands-on mason who has specialised for many years on the memorial side of the stone industry. He was Technical Advisor to the National Association of Memorial Masons, writing manuals and City & Guilds qualifications. If you have an issue regarding any aspect of memorial masonry that you would like help with, David is here for you. Send your queries and comments to David at [email protected].
This month I am turning my attention to memorials that change colour.
For some time there have been complaints from grave owners that their granite memorials have either changed colour or become blotchy. Most of the problems occur on green or black materials.
Granite is an igneous rock that is crystalline in structure. To be a granite, the material must consist of as least 65% silica and contain mica and feldspar. The colours used for memorials are from black to light grey, pink to red, blue and green.
All these materials show at least a small sparkle when polished. The dense black is not true granite as it is missing one of the elements that make it geologically granite. Nevertheless, it is normally included in the granites and it has become increasingly difficult to find. ‘Best Ebony’ used to be the blackest but even this seems to have a slight browny hue these days.
The traditional granites used for monuments are quite dense and do not suffer badly from severe environmental conditions. When they are polished they can usually be brought back to a near original finish with careful cleaning.
Open crystalline granites are especially susceptible to discolouration because they draw in pollutants from the air as well as cement or resin used in fixing by capillary action. Any staining is hard to remove as the crystals stain with the dirt.
Many memorials these days come from India or China. India has some particularly dense black granite and China has a multitude of colours.
Polishing has sometimes been a problem and some of the suppliers have found inventive ways to make the granite shine. A waxy or oily smell sometimes emanates from the stone, which gives a clue.
A concoction of applied finishes have been used and not all of them are particularly resilient when they are stood up in a cemetery and exposed to the British climate. Some of these granites also seem to be more porous than those from Scandinavia and South Africa.
One of the green granites from India has a metallic element embedded within it that oxidises in the air. Customers complain the memorial has gone rusty. In the trade it is called bronzing. It becomes apparent when darkening applications used to cover the marks within the material weather off.
Any porous material used in this country can a problem.
The problem has been worsened by memorial masons buying memorials directly from parts of the world that offer a good price. They might import a whole container, sight unseen, instead of ordering through a wholesaler who has face to face contact with the suppliers and sees what he is buying before it is shipped.