Guidelines for Determining Appropriate Cleaning Methods

The appropriate cleaning method depends on the purpose of the cleaning and on the extent of the work to be done. A guide to assist in the selection is shown in the table, the principle of which was developed in a document published by the National Trust. The table uses four variables:

• I_R degree of resistance of the masonry to damage
• I_T tenacity or degree of difficulty of removal of the soiling
• I_S severity or potential of the method for damaging the masonry
• I_C cleaning ability of the method.

When selecting a cleaning method, it must be recognised that the removal of stains may alter the physical condition and appearance of the masonry. The task is to select the method that achieves the desired degree of cleaning while resulting in the least damaging alteration to the masonry.

The table can be used as a general guide in the following way.

• Identify the masonry material (I_R) and the tenacity of the soiling (I_T)
• Select a number of provisional cleaning methods such that the cleaning method (I_C) must be as high as the tenacity of the soiling (I_T)
• Select a method with a severity (I_S) that does not exceed the masonry degree of resistance (I_R).

As an example, consider the removal of soft urban grime (I_T = 3) from smooth coloured masonry units (I_R = 3).
Cleaning methods which have I_C = 3 or 4 are considered and low-pressure water jetting is selected (I_C = 3).
This has an I_S of 2 to 3 and therefore should not damage smooth coloured masonry with an I_R of 3.
The method should then be tested. If not successful, a method with higher I_C should then be considered. However, if high-pressure water jetting with acid were selected (I_C = 6) then this has an I_S of 7 and would probably damage masonry with an I_R of 3.

Masonry Unit Type Index of resistance to damage, IR	Soiling Index of tenacity, IT	Cleaning Method Index of severity, IS (IS should not exceed IR)	Cleaning Method Index of cleaning ability, IC (IC to be as high as IT)
IR = 0	IT = 0	IS = 0	IC = 0
Lime mortar. Lime mortar, hard. Cement render, old.	Loose dust. Climbing plants, moss. Fungi, algae. Loose flaking paint. Powder efflorescense.	Dry Brushing. Water misting. Very low pressure water washing. Organic Solvents. Steam cleaning.	Dry brushing. Very low pressure washing.
IR = 2	IT = 2	IS = 2	IC = 2
Lightweight units. Composition Mortar. Smooth, coloured units.	Deposited efflorescence. Oil, Grease. Soils. Soft urban grime.	Hand brushing. Hand brushing + detergent. Low pressure water. Mechanical scrubbing. Chemical. Bleach.	Steam Cleaning. Organic solvents. Low pressure water washing. Hand brushing.
IR = 4	IT = 4	IS = 4	IC = 4
Standard Units. Fire rated units.	Bird droppings. Plaster. Tar. Old hard paint. Soft mortar smears. Timber stains. Iron & welding splatter.	Medium pressure water jetting.	Medium pressure water jetting. Mechanical scrubbing with detergent Chemical (acids). High pressure water jetting.
IR = 6	IT = 6	IS = 6	IC = 6
Honed Units. Polished units.	Moderate urban grime. Hardened mortar smears.	Manual abrasive removal. High pressure water jetting. Very high pressure water blasting. wet sand blasting	Very high pressure water blasting.
IR = 8	IT = 8	IS = 8	IC = 8
Shot blasted units. Split faced units.	Modern plastic paints. Hard urban grime.	Dry sand blasting. Mechanical grinding.	Wet sand blasting. Dry sand blasting.
IR = 10	IT = 10	IS = 10	IC = 10