LIMESTONE

Stone is a material that man has used in building for several thousand years and it still lends itself to modern usage and design, and limestone in its various geological forms is probably the type of natural stone most widely used for this purpose. It occurs naturally in many parts of the UK with the limestones of the Jurassic period being those of most importance in this field. They run broadly in a belt from Dorset and Somerset in the south west, through the Cotswolds and Oxfordshire, on into Northamptonshire and Lincolnshire, to just north of the Humber.

Limestone is a sedimentary rock. Many of the commonly used varieties were formed by the accretion of the hard remains of former organisms such as corals and shells. These materials principally comprise calcium carbonate (calcite), as does the cement. Calcite is a relatively soft mineral and careful selection will be needed when you choose this for use as flooring. Variations in the types and quantities of shell and other remains and the nature of the cement provide a huge range in the types of limestone available.

The limestones of these areas were used for building in Roman times and, in later centuries and to-date, have been the traditional building material of their localities, Many buildings throughout these counties bear witness to this. In addition, some limestones are extensively used outside their areas of origins as is evidenced by the many fine buildings to be seen throughout the British Isles, particularly in major cities and towns. Cathedrals, churches and numerous public and private buildings provide outstanding examples of the durability and beauty of these limestones and they show through the skill of the mason, how this stone is so eminently suitable for producing masonry of excellence in all aspects of plain and detailed work that may be required. Among the better known limestones of the Jurassic age are those of Purbeck, Clipsham, Ancaster, Ketton, Bath, Doulting and Weldon, whilst Portland stone is a characteristic feature of London architecture. From the earlier Carboniferous age come the limestones of Derbyshire and Cumbria such as Hopton Wood, Sheldon, Orton Scar and Salterwath.

Imported limestones, most notably French, German, Portuguese, Spanish, Turkish, Egyptian and Israeli are also available in the UK.

Appearance

The colours of limestones range from almost white to the warm honey tone of Bath stones with grey/blue and darker browns a feature in some limestones. Textures range from fine even grained stones to the smooth fossil bearing types and to the coarse open textured Portland roach. Certain limestones such as Purbeck and Hopton Wood take a polish and can be used as “marbles”.

Applications

Limestones are used extensively in both new building and restoration where their ease of working facilitates the production of cladding, ashlar and other forms of walling. They are also suitable for flooring.

Many limestones are particularly suited to carved and moulded work. Components such as cills and jambs, soffits and copings, heads and mullions are readily produced to add interest and enrichment to building facades.

Performance

Limestones generally have good load bearing properties and weathering characteristics although correct detailing is important as with all building materials. Limestones must not be used above sandstones on exterior elevations.

The properties of individual types of stone can vary considerably and advice should always be sought on the selection of suitable stones, both for general and particular applications.

MARBLETrue marbles in the geological sense are metamorphosed limestone and are principally composed of recrystallised calcite formed into an interlocking granular structure. Some hard or partially metamorphosed limestones are incorrectly referred to as marble, even though they exhibit many of the characteristics of marble.

In its ‘purest’ state marble is typically white, however the presence of other minerals can often provide colour(s). In some cases the colour appears as irregular shaped patches (brecciation) or as substantial veining. Specialist advice should be obtained as to the suitability of such marbles in particular locations.

Greater care must also be taken during installation as other construction defects easily taint the pureness of marble, particularly from underlying materials, and may result in staining at a later date.

It is common to use marble with a highly polished surface finish and, as a consequence, slip resistance should be carefully considered. Marble is imported.

QUARTZITES

Quartzites are typically the metamorphosed product of an original sedimentary rock e.g. sandstone, composed almost entirely of quartz.

In general quartzites are hard wearing, have low porosity, a high compressive strength and good durability making them suitable for use as flooring. The metamorphic varieties in particular may be brittle and so some care is required in assessing a material prior to use. While composed largely of quartz, the presence of some impurities can lead to the development of different colours and the typical range is from white to yellowish brown, through to green, brown, gold, grey and blue. In Eire, the metamorphic quartzites have a complex geological history which has tended to result in them having a relatively small natural block size. All operating quartzite quarries in the British Isles occur in Eire. Elsewhere quartzites are commercially available from Norway, Sweden, Italy, South Africa, USA and Brazil.

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