Technical Data Sheet
Caithness Flagstone

A & D Sutherland Ltd

Spittal Quarry, Caithness, KW1 5XR

Compiled May 2000

This data sheet was compiled by the Building Research Establishment (BRE). It is base on data current tests at BRE (2000). The data sheet was compiled in May 2000. The work was carried out by BRE as part of a Partners in Technology Programme funded by the Department of the Environment, Transport and the Regions and A & D Sutherland Ltd and does not represent an endorsement of the stone by BRE.

General 

The quarry is near Thurso, Scotland and has been worked for over a century; Traill of Olrig/ Rattar in 1824 started Castlehill quarry and the first shipments were made in 1825. The present owners have owned the quarry since 1966 and there are plenty of reserves of stone.

Stone is extracted from a 3m high bed which lies under 4.5 - 9m of overburden. Caithness Flagstone is quarried from 16 principle layers split into a total of 49 individual flagstones, these range in thickness between 20mm and 80mm approximately. Traditionally the stone is worked by splitting it into flags by hand; the resulting flags have been shipped all over the world.

A set of Bronze age standing stones in Caithness flag are still standing at Achavanich (ND188417). There are also megaliths at Brodgar and Stenness.

Petrography  
Caithness flagstone is from the Old Red Sandstone of Devonian age. It is very fine grained with a dark grey colour which weathers to a fawn.

Expected Durability and Performance 

It is important that the results from the individual tests are not viewed in isolation. They should be considered together and compared to the performance of the stone in existing buildings and other uses. Sandstone is traditionally acknowledged as generally being a very durable building and paving stone and has been used extensively in many towns and cities in the UK.

Caithness flagstone appears to be a durable stone that has limited resistance to acid rain or air pollution. In addition, the slight weight loss in the sodium sulphate crystallisation test (both 14% an saturated) indicates excellent resistance to salt damage (for example in coastal locations or from de-icing salts). From the frost test the stone should also have good frost resistance. The flexural strength of the stone is extremely high for a sandstone in comparison with many sandstones. The strength indicates that the stone should be suitable for use in heavily trafficked areas.

Overall, Caithness flagstone should be suitable for use in many aspects of construction including flooring, paving, load bearing masonry and cladding. The stone is suitable for areas where a long service life is needed in an aggressive salty environment (e.g. the harbour at Castletown near Caithness) with the exception where acidic conditions may prevail. The stone has been used predominantly for paving and flooring, though is has been used in staircases, sills and wall cladding.

Test Results - Caithness Flagstone

Safety in Use
Slip Resistance (Note 1)	Wet: 62	Values > 40 are considered safe
Abrasion Resistance(Note 1)	Not Tested	Values <23.0 are considered suitable for use in heavily trafficked areas
Strength under load
1) Compression(Note 2)	Not Tested	Loaded perpendicular to the bedding plane- ambient humidity
2) Bending (Note 1)	37.2 Mpa	Loaded perpendicular to the bedding plane- ambient humidity
Porosity and Water Absorption
1) Porosity (Note 3)	0.3 %
2) Saturation Coefficient (Note 3)	0.98
3) Water Absorption	0.1 % (by wt)
4) Bulk specific gravity	2684 kg/m3
Resistance to Frost
Flexural strength after Freeze/Thaw Test (Note 1)	39.2 MPa	Loaded perpendicular to the bedding plane - ambient humidity
Resistance to Salt
Sodium Sulphate Crystallisation Test (Note 3) (Saturated)	0.0% Mean wt loss
Sodium Sulphate Crystallisation Test (Note 3)	0.05% Mean wt loss
Resistance to Acidity
Acid Immersion(Note4)	Fail

Test methods 

Note 1 = prEn1341, Note 2 = prEN 1342, Note 3 = prEn 1341 /BRE 141, Note 4 = BRE 141 

All tests were carried out at BRE in 2000

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