In this new, regular series for 2008, consultant Barry Hunt answers reader enquiries

If you have a question, email it to: nss@qmj.co.uk marked for Barry\'s attention

Q: In the course of cooking using the spice turmeric a customer has stained their granite kitchen worktop and nothing seems able to remove the stain. Is there anything I can do?

Turmeric is the powdered rhizome or underground horizontal stem of Curcuma longa, a plant related to ginger. The yellow colour is caused by curcumin, which makes up around 5% of the powder and which seems able to stain even the most apparently non-staining surfaces.

Curcumin is not soluble in water, which is why it is so difficult simply to wipe away using a damp cloth.

It is, however, soluble in organic solvents such as methylated spirit and acetone. But before employing chemicals to any stone surface a small and unobtrusive test area should be carried out to determine if such chemicals have any undesirable effects.

If the stone was properly sealed the turmeric stain should not have penetrated and will probably be sitting in microscopic surface pits and should be easy to remove. If the stain is stubborn it may have penetrated the stone, which can occur with even the most apparently dense and hard stones because of the relatively small molecule size of curcumin. Such stains may be impossible to remove with cleaning chemicals because they cannot get into the stone to react with the offending material. However, there is one possible treatment: sunlight.

Curcumin breaks down in visible and ultraviolet light and in a well lit area stubborn stains should fade over time. If you cannot wait, the process may be speeded up by the application of concentrated light. Curcumin is unaffected by heating and this should not be attempted.

Under most circumstances organic solvents will not affect the stone but might attack or strip some surface treatments that should have been used. If a treatment is working properly, water droplets should sit up high on the surface in the same way as they do on a waxed car. If they do not, the stone may not have been sealed properly and sealing should be carried out at the earliest opportunity to reduce the potential for future staining. At this point it would be appropriate to seek professional advice from Stone Federation members undertaking such work.

Finally, if you are using turmeric regularly it may be worthwhile trying to reduce the risk of spillage on to work surfaces by using pans with higher sides to prevent spitting. Any spills that do occur should be blotted immediately by laying down kitchen paper or a dry cloth to prevent spreading.

Reference

Natural Stone Kitchen Worktops. A code of practice for the design, manufacture, installation and maintenance of natural stone kitchen worktops, including reception desks, counter tops and other similar horizontal and associated vertical surfaces, for domestic and commercial use, published by Stone Federation Great Britain, February 2005 and available from the Federation.

Barry Hunt is a Chartered Geologist and Chartered Surveyor who has spent 20 years investigating issues relating to natural stone and other construction materials. He now runs IBIS, an independent geomaterials consultancy undertaking commissions worldwide to provide consultancy, inspection and testing advice. Tel: 020 8518 8646

The advice offered in answer to readers\' questions is intended to provide helpful insights but should not be regarded as complete or definitive. Professional advice should always be sought with all stone-related issues as each problem will inevitably have its own unique set of circumstances.