SANDSTONE

The term sandstone is used to describe almost any stone of sedimentary origin with a granular texture. Some other types of stone that may fall under this classification include gritstones, siltstones, greywackes, conglomerates and marls. Aside from particle size variations, the dominant factor affecting sandstone performance is the grain cement that may be siliceous, calcareous, clay bearing or iron-rich.

Geologically it is a sedimentary type rock made up of various mineral particles mainly quartz, mica and felspar. All these minerals are bonded together with natural cements such as silicas, calcium carbonates, iron oxides and clays. The individual make up of each sandstone gives a wide range of colours. A pure silica sandstone is white with the colours arising from other minerals. Iron oxides can cause the stone to vary from buff or brown through to the deepest red. Grey colour stone is due to the presence of clay and green could indicate the presence of glauconite which contained potassium.

Sandstone is a natural material traditionally regarded as the building stone of the North. However, sandstone can be seen throughout the United Kingdom in many prestigious buildings from structures which form part of our heritage to modern office blocks.

Sandstone is traditionally used as a building material in many areas where it occurs locally particularly in Derbyshire, Yorkshire and the North East of England, Wales and Scotland. Many of the “stone cities” of the North derive their particular architectural character from the use of sandstone.

Most of the major sandstone quarries working today are located in the North of England, Derbyshire and Scotland. For flooring purposes, sandstones are also imported.

Appearance

Sandstone has many colours ranging from white, grey and buff to various shades of pink and red.

The colour and texture of stone is self evident, however the colour of dry stone can change quite dramatically on wetting. Natural weathering can play a major part in the change of colour. Such colour change can sometimes be viewed on abandoned faces of the quarry although it is much more important to view existing buildings constructed from the same stone.

Sandstones generally have an even texture, which may vary from course to fine, but the stone from some quarries also show attractive natural markings.

Applications

Many types of sandstone are suitable for paving or flooring. Some can be split (”riven”) otherwise the normal finish is sawn. Tooled, sandblasted and flame textured finishes are also available.

There is a vast variety of surface finishes available from the traditional rubble walling to wrought stonework with its finishes to different forms ie. boasting, pitched, broached, hammer dressed and rusticated, to the very intricate looking vermiculated. But most importantly, credit must go to the craftsmen who hue the sandstone to all the various forms from large stones with a high degree of moulding to the most intricate of carving work. Sandstones are suitable for blcok construction in most building and engineering work and can now be cut to relatively thin sections for cladding to steel framed or concrete structured buildings.

All in all sandstone is one of the few historic natural minerals still in use today. It provides excellent durability and is virtually maintenance free. It is aesthetically appealing to the eye whilst giving great value for money to the client.

Performance

Most sandstones have good load bearing and weathering properties. Correct detailling with regard to weathering is of prime importance, especially on buildings designed with sloping surfaces and without overhangs. Great care is required in the design of non-ferrous cramps and corbels for fixing to structures.

SLATE

Slate is found extensively, throughout the British Isles and has, for centuries, been a major source of building stone. Historically, and, in common with other stones, it was first used in and around the immediate areas of availability.

The term slate is often used to describe any rock that can be easily split into thin sheets, principally for roofing purposes. True slate is defined by the presence of a ’slaty’ cleavage; this allows the slate to be split at almost any point through the stone parallel to the cleavage plane. Most true slates are metamorphosed sediments, often formerly mudstones; however, some British “slates” are derived from volcanic ash sequences and are not true slates in the strict geological sense.

Appearance

The major UK sources of slate, with their distinctive colours, are the Lake District blue/grey, light green, olive green and silver grey, North Wales blue, grey, blue-black and red, and Cornwall – grey.

Slate is easily split (”riven”) into thin sections, giving a natural finish. Other finishes include sawn, sanded, fine rubbed, flame textured, bush hammered and water jet.

Applications

From the very early, basic applications, British Slate is now used for very wide and diverse purposes. Modern quarrying and machine processes allow prime blocks to be sawn or split into large slabs and a variety of thicknesses. Current products include: roofing, cladding, cills, copings, flooring & paving, treads, plaques & memorials, worktops & fireplaces.

Slate is typically a very durable construction material and rarely exhibits visible degradation, even when placed under extreme atmospheric conditions for many years. For flooring purposes U.K. slate is unlikely to delaminate or exhibit efflorescence.

High flexural strength and typically very low porosity makes slate eminently suitable for flooring purposes and it requires little maintenance.

Caution should be exercised when choosing material marketed as slate as some may not be suitable for flooring. If any doubt exists about the material being considered, specialist advice should be sought.

Very thin, split, natural riven sections are used for roofing and the same methods produce thicker material for flooring and cladding. Slateis readily available in sawn, fine rubbed and other finishes wuch as sand-blasted and flame textured.

Performance

British Slate has a proven record of durability and being supplied fit for its intended purposes. This is readily seen on countless buildings in the UK and many areas of the world. Such examples of installation are witness to the ability of slate to withstand the rigours of extreme atmospheric conditions. Slate is extremely stable and will not degrade, warp or twist. Quality Assurance, management systems are widely used throughout the industry.

More Reading: Part 1, Part 2, Part 3, Part 4