Scratching the surface (not)

Published Thursday 27th January 2011

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

The results of the study are now helping a Leeds-based SME to convince sceptical clients of the merits of sand-free cleaning.  Since commissioning the report, WGL Stoneclean's workforce has tripled and its monthly sales have increased tenfold.

The novel 'Steamacc Process' offered by WGL Stoneclean 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 150 degrees centigrade. 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 the University of Leeds 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, SEM revealed that the acid only removed a small amount of cement- less than one grain's depth of material (less than 100 µm) - 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: "This is a great demonstration of how academic expertise can be put to good use to help local a business. Our work with the high-resolution microscope gave WGL Stoneclean the scientific proof they needed to convince clients that Steamacc cleaning was safe to use and has had a positive impact on their order book. We would definitely like to do more of this type of consultancy work. "

Bernard Coleman, Director of WGL Stoneclean, said: "The figures presented in the report indicate that even if we repeated this cleaning process every five years, it would take 7000 years to 'wash' a building away. We now believe that the Steamacc process is the least damaging, most cost effective and environmentally friendly method of cleaning stone buildings that is available in the UK market."

The consultancy work was funded by a voucher scheme run by the Government-funded agency Business Link Yorkshire.

For further information
Paula Gould, University of Leeds press office: t: 0113 343 8059, e: p.a.gould@leeds.ac.uk

Notes for Editors
WGL Stoneclean Ltd was originally founded in the early 1970's before being purchased by the current management in 2004. Over the past six years the company has cleaned some high profile and listed buildings and has worked for over 70 of the Northern Region's and Country's leading contractors. Amongst the buildings that WGL Stoneclean Ltd has cleaned and restored are: Leeds Grand Theatre; The Archbishop's Palace, Bishopthorpe, York, Wakefield County Hall; Leeds College of Art; Bootham  School York - this project won the 2009 York Design Award; Victoria Hall, Saltaire, High Royds Hospital, Menston; Roberts Wharf, Leeds etc.