Safe stone cleaning solution

07 March 2013

Pioneered by WGL Stoneclean in Yorkshire, the 'Steamacc' process is a stone cleaning method with a difference. It has recently been assessed by the University of Leeds' Department of Civil Engineering with surprising results

Pioneered by WGL Stoneclean in Yorkshire, the 'Steamacc' process is a stone cleaning method with a difference. It has recently been assessed by the University of Leeds' Department of Civil Engineering with surprising results

The report has concluded that the process is effective, environmentally friendly and safe, with damage limited to between only 5-150 microns of the stone's surface, says WGL Stoneclean.The Steamacc (steam, acid, carbon, cleaning) technique uses a combination of super-heated steam and mild acid and has successfully been used by WGL Stoneclean for the last four years.The acid solution is applied using a low pressure sprayer and then rinsed off within seconds using the hot steam.

"The use of high energy water combined with a traditional acid treatment has resulted in less surface damage, lower acid concentrations, faster treatment times and the virtual elimination of acidic wash," says Bernard Coleman,WGL Stoneclean director.

While many architects and conservationists assume the use of acid in stone cleaning can be harmful, this report shows that when used properly and with skill, the result is an impressive one.

"The combination of techniques WGL has pioneered is probably the most important step forward in masonry cleaning in five years," claims Coleman."It's an area of construction activity that's under researched, and environmental pressures require these cleaning methods to be quantified.

"For example,we are aware that abrasively blast cleaning, even if the system is pressure regulated, can remove 1-3 mm of the stone's surface, whereas the university report indicates erosion of the surface from this procedure, the Steamacc, to be only 5-150 microns." The process is used primarily to remove carbon staining. Some of the UK's most impressive architectural heritage stands today only as a shadow of its former grand self.Carbon staining has led to many buildings blackening over time from industrial periods in history which subjected properties to intense pollution.

Over time industrial pollution forces small carbon particles into the stone structure through wind, rain and temperature differences.The particles cannot merely be washed away with water. One early approach was to sand blast the stone.This was a quick, but damaging technique as the first 1-3 mm of the surface were removed.And, more worryingly, this method produced dangerous silicosis debris.

Subsequently, chemicals started to be used in the cleaning process to open up the surface of the stone's structure.

However,many of these cleaning solutions contained high (>20%) concentrations of Hydrofluoric acid (HF). The development of the Steamacc process has resulted in a combination of using low concentrations of HF and lower dwell times, which ultimately means less damage.

Under the microscopy

In the university report, four samples of Yorkshire Stone were tested using scanning electron microscopy which allowed a greater level of magnification.The first sample was taken from the centre of the stone to show it in its original, untreated or cleaned state.The further three samples were cleaned using the acid solutions of varying strength and studied to analyse the level of stress to the stone caused by the acid.

The original sample showed how the stone is comprised of microscopic angular sand grains, about 200 microns in diameter with a fine cement holding grains together.

"Optical microscopy has shown that the cleaning process used by WGL Stoneclean effectively removes the black/grey patina formed on weathered stone samples," comments Dr Leon Black, the report writer.

"Scanning electron microscopy has looked a little more closely at the surfaces of pristine,weathered and cleaned stone samples and shown that the solution used to clean the stone does not damage the stone when used at low concentrations.

"Work is being conducted to assess the extent of material loss upon cleaning of the stone, but from these results, it does not appear as though undue distress is caused to the stone surface when cleaning with dilute or mild cleaning solutions." WGL has used the Steamacc process on several high profile properties in the north of England, including the Archbishop's Palace in York, Leeds Grand Theatre and Wakefield County Hall.

Contact the company for a copy of the report or further information.