Researchers test WGL’s new stone-cleaning technique

Historic houses across Yorkshire that are in need of a facelift may not have to be sand-blasted in future, sparing them from possible surface damage.

Researchers from Leeds University have shown that an alternative method of industrial cleaning, using diluted acid and super-heated steam, can be used safely on the ancient stonework.

Now, the so-called ’Steamacc process’ technique is being offered by WGL Stoneclean, a stone-cleaning company in the north of England.

The process involves spraying blackened stonework with a solution of dilute acid and then washing the surface with a high-pressure jet of super-heated steam at 150oC. The acid essentially opens up the surface of the stone, allowing the steam to penetrate between and behind individual sub-millimetre-sized grains. Deposits of soot and dirt are then forced out by the power of the high-pressure wash.

The traditional method of cleaning stonework by shot-blasting the surface with sand, grit or chalk is effective but, owing to its abrasive nature, can damage the stone grains.

WGL Stoneclean asked civil engineers from Leeds University to analyse the surface of stone samples before and after Steamacc cleaning with three different strengths of the dilute acid wash. The researchers used conventional optical microscopy and a high-resolution scanning electron microscope (SEM) to look at samples of Yorkshire stone removed from buildings and a further set of samples prepared in the laboratory.

Images from the optical microscope confirmed that the process cleaned away the black-grey patina from the weathered York stone effectively. But, more importantly, the SEM revealed that the acid only removed a small amount of cement — less than one grain’s depth of material (less than 100µm) — from between the stone grains. The stone grains themselves were not damaged at all when the acid was used at low concentrations, and only minimal damage was seen at higher concentrations.

Dr Leon Black, who led the study, said: ’Our work with the high-resolution microscope gave WGL Stoneclean the scientific proof it needed to convince clients that Steamacc cleaning was safe to use and has had a positive impact on its order book.’

Bernard Coleman, director of WGL Stoneclean, added: ’Even if we repeated this cleaning process every five years, it would take 7,000 years to “wash” a building away. We now believe that the Steamacc process is the least-damaging environmentally friendly method of cleaning stone buildings that is available.’