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T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 6

T W E N T Y T H I R D E D I T I O N

SERV I CES & TREATMENT :

PROTEC T I ON & REMED I AL TREATMENT

4.1

PROPERTIES OF ABRASIVES

Abrasives may be chemical in composition,

but their effect in cleaning is normally physical

and results from impact or movement while

in contact with the substrate. Abrasives are

typically quarried as a primary product and are

thus natural (calcite or olivine for example),

or manufactured and thus synthetic (such as

calcium silicate – a by-product of steelmaking

– or recycled bottle glass). A small number

are manufactured specifically for the purpose

but all abrasives require significant work to

produce or refine them. For this reason, in

a factory environment, efforts are made to

recycle them, either by centrifuge or magnetic

recovery (of steel shot), to separate undersized

particles and debris. For masonry cleaning this

is usually impractical as the abrasives are often

used wet and are relatively soft, breaking down

considerably with each use.

Both natural and synthetic abrasives can

be classified by their size, hardness, shape

and density. Other active characteristics

may include solubility (sodium bicarbonate),

absorbency (sodium bicarbonate and abrasive

impregnated sponge), temperature and the

ability to turn from solid to gas (dry ice).

Size

Traditionally, particle size distribution

has been determined using laboratory

sieves but it is now often measured using

‘laser diffraction’. This measures the range

and proportion of particle sizes within a

representative sample. A good abrasive

will have a narrow range of particle sizes.

The approximate mean particle diameter is

sometimes used to describe the sample as a

whole, for example 150 microns (0.15mm).

For sensitive work, preference should be

given to abrasives of smaller particle size, all

other factors being equal. For micro-abrasive

work the size might be in the range 10-100

microns while that for larger-scale work

might be in the order of 80-250 microns. Sizes

larger than this are normally reserved for

industrial applications or ‘surface preparation’

– the removal of all deleterious material

and provision of surface texture to aid the

adhesion of a specialist coating or render, not

a conservation application.

Hardness

This is determined by a scratch resistance

test which provides a value between 1 and

10, recorded on the Mohs Scale of mineral

hardness, with higher readings indicating

greater hardness. A copper coin for example

has a hardness of approximately 4 Mohs

while that of window glass is 6 and diamond

is 10. An abrasive of one particular type will

have a relatively consistent hardness from

one particle to another, but the same is not

always the case for the substrate. Granite for

example is composed of a range of minerals

including quartz (7), mica (3) and feldspar (6).

A siliceous sandstone may comprise grains of

silica (quartz: 7) cemented with silica, while

a calcareous sandstone may be composed

of silica (7) bonded with calcium carbonate

(2.5–3).

For polished surfaces and un-weathered

concrete, the abrasive hardness should

be less than the softest constituent of the

substrate. For glass, or glazed surfaces (such

as terracotta/faience), the value should be at

least 2 Mohs less than the substrate. If this

cannot be achieved it may be necessary to

adopt a different cleaning technique. These

examples aside, abrasive selection for open-

textured substrates is more usually dictated

by the hardness of the soiling or coating to

be removed – in other words the abrasive

should be similar or harder than the unwanted

matter, even where the substrate is softer.

This may at first seem counterintuitive, but

prolonged use of a soft abrasive on a hard

coating over a soft substrate will invariably

result in pitting compared with a hard/sharp

but fine abrasive.

Shape

The shape of an abrasive particle can be

described as rounded, cuboid or angular.

Particles with more and sharper edges

normally have increased cleaning efficiency.

Sharpness is often, although not always,

associated with minerals of greater

hardness. As a general rule, rounded

particles are likely to be more conducive

to retaining patination (including bronze

for example) than sharp abrasives.

Density

Higher density permits more energy to be

transmitted by a particle at otherwise the

same velocity. Copper slag for example has

both great hardness and a high density.

For masonry cleaning, high density is not

usually necessary and can cause damage.

ABRASIVE TYPES

BS8221

offers a list of abrasives that may be

used for dry air abrasion and a truncated list

for ‘dry micro abrasion’. English Heritage’s

Practical Building Conservation: Stone

(see

Further Information) offers an alternative list.

There are certain anomalies but in general, for

cleaning masonry, those >7 Mohs should only

be used at very small particle size (principally

for micro abrasion) while those ≤2.5, especially

when of low density, can be used at moderately

large particle size. Those in common use for

careful cleaning of masonry fall generally into

the 2.5–6.5 Mohs range of hardness.

It is vital, however, to consider all the

abrasive characteristics, not just hardness. All

abrasives are an inhalation risk, particularly

at fine particle size. Many will be marketed

by their trade name rather than chemical or

descriptive name. Product material safety data

sheets or technical data sheets will normally

provide the definitive description. Aluminium

silicate (coal slag), calcium silicate (blast

furnace slag) and copper slag are all forms of

A swirling vortex

abrasive system:

compressed air is

dried by the air cooler

before it arrives at the

pressure vessel or ‘pot’.

Abrasive is metered

into the air flow and

carried towards the

nozzle. Pressurised

water is separately

directed to the nozzle

where it can be

added to the swirling

mixture via a valve.

MATERIAL

HARDNESS

(Mohs)

SOURCE/NOTES

Talc

Crushed egg shells

Sodium bicarbonate

2.5

2,3,† (BS states 4.0 Mohs)

Crushed almond shells and olive stones

2.5–4.5

1,3

Crushed calcium carbonate (normally carboniferous limestone)

2.5-3

1,2,3,†

Marble (crystalline calcium carbonate)

†

Synthetic aluminium oxide (alumina)

3.5

Dolomite

3.5–4

3,†

Glass bead

4-5

1,2,3

Calcium silicate

5.5–6.5

1,†

Crushed glass

5-6

2,†

Mineral slag

5.5–8

1,2 (BS states 7.0–8.0 Mohs)

Aluminium silicate

6–7

1,†

Crushed stone (e.g. olivine)

6.5–7.5

1,2,3

Copper slag

7.5

1,3

Silicon carbide (carborundum)

2,3 (BS states 7.0–8.0 Mohs)

Natural aluminium oxide (corundum)

1,2

Source:

BS8221

, dry abrasion

BS8221

, dry micro abrasion

English Heritage,

Practical

Building Conservation: Stone

Notes:

†

In common use for historic masonry cleaning