In response to this column in December about the white marble floor turning yellow I had several emails from marble folk, advice from an eminent geologist and contact from an architect who seems convinced I would be a useful addition to his team on a large refurbishment project using lots of white marble. I am looking forward to working on that project.

The advice from my geologist friend centred around the use of UV light as a germicidal treatment for the floor.

The theory is that some of the resin used to glue the floor joints may be the source of organic growth.

He’ll be glad to know I’m following his advice.

He thought that as well as the possibility of the staining being caused by the oxidization of pyrites within the ground mass of the stone, it might also be possible that some of the yellowing could be attributed to fungi or bacteria growing on the back of nutrients from the cement and organic materials. He also thought they could possibly be growing on the soluble components of the resins being used.

The other issue is that alkaline water can attack resins to release some of the materials that are then fed on by the organisms.

Germicidal UV light is commonly used to sterilize and kill bacteria in hospitals, laboratories and food processing plants, so why not stone?

Limited penetration perhaps but non-invasive, unlike sodium hypochlorite (NaOCl) – or bleach, as it might be more commonly known – often used to remove various stains. Unfortunately, sodium hypochlorite also dissolves the marble if used too frequently and long term.

By carrying out isolated and controlled UV light tests on selected parts of the floor it might be possible to establish if some of the yellowing is as a result of bacterial growth.

If the cause of the yellow staining is organic, the UV light will kill the bacteria.

At least, that’s the theory.

So... I have hired the largest hand-held germicidal UV light I could find. I have to wear a fully protective body suit, gloves and welder-like head gear.

The unit can reach 100ºC and can only work for 10 minutes at a time to avoid damaging the bulb – enough time to roast the family pet or set fire to the house.

I have just finished reading the health & safety instructions that came with the lamp. If there’s no article next month check the obituaries.

The intention (should the family pet, the house and I survive the initial testing) is to return in a week to see if control areas have changed.

Even if the lamp kills the bacteria, it could take further cleaning to remove the yellow staining. Then we’ll just wait to see if it returns.

Meanwhile, in a slightly more conventional approach, we await the results of SEM + microanalysis of core samples taken in December in the hope that these will reveal more about the causes of the staining.

What is clear, as the contractor keeps telling me, is that the marble definitely did not look like it does now when it was purchased, nor when it was first installed. As pure as the driven snow, he insists.

As my friend the geologist has said on many an occasion (and I quote): “It’s never the stone’s fault; it’s always what we do to the stone that causes all the problems.” At least, I think that’s what he said.

There is a story in there somewhere about the family pet and not eating yellow snow, but I can’t for the life of me extract it. Let’s hope we have more success with the yellow marble.

I’ll keep you posted.

Robert Merry, MCIOB, is an independent Stone Consultant. He ran his own stone company for 17 years before becoming first an independent project manager and now a consultant. He is also an expert witness in disputes regarding stone and stone contracts. 0207 502 6353 / 07771 997621. [email protected]