Ask the Expert : Wood lacquer stains limestone
Q: A lacquer I applied to my woodwork has turned the adjacent limestone a vivid blue colour. Can I get rid of this?
There will be hydrocarbons and alcohols present in the lacquer, although these are not likely to have caused the observed effects. However, lacquers often also contain organic acids in the hardening agents such as p-toluenesulfonic acid. I suspect an organic acid has reacted with the limestone and the by-product is this blue discolouration.
The fact that the discolouration is blue suggests that a type of copper mineral might have been formed – cobalt and other rarer blue compounds are less likely. This could have occurred if there are copper traces in the limestone – and copper is often found in limestone in forms such as chalcopyrite and cuprite.
The sulfur component of an organic acid compound, such as
p-toluenesulfonic acid, might actually be combining with the copper to form copper sulphate hydrate, which certainly exhibits an intense blue like that shown in your photograph.
Copper sulphate-based materials can typically be relatively insoluble, although their appearance may be altered by heating to remove some of the hydrates present.
At temperatures of around 150°C, copper sulfate hydrate typically turns from blue to white. So you could try heating the stone surface to see if the discolouration can be toned down.
Assuming this works, the stone would need to be thoroughly sealed to prevent the copper sulfate rehydrating and turning blue again.
The only problem with heating the stone is that it might crack, although localised intense heating with a source that does not leave carbon smuts might avoid this.
The alternatives are trial and error using different chemical cleaning combinations, or simply grinding the surface. As the stain appears to have affected the outermost surface it is possible that fine rubbing and brushing could actually remove much of the effect. It is probably worth trying first. If it doesn’t work, try it in combination with localised heating.
If you end up resorting to chemical cleaners, there is a variety of combinations that might work. One I have seen recommended for limestone is a combination of cyanides of sodium and potassium.
It is difficult to be more specific on this matter without fully understanding why the discolouration has occurred and what it actually comprises. We may never know. However, a simple, practical and methodical approach to solving the problem may mean that for this exceptional circumstance we do not need to know.
We are forever increasing the number of chemical products that may work without problems in isolation but might lead to unexpected results in combination. It is usually worthwhile applying any treatments that might come into contact with stone to a spare piece of the stone to see if there are any obvious effects before using it.