Types of Stone in Period Houses

Identifying the type of stone you have is important in understanding its properties and how to care for it.

Each rock comprises one or more minerals and it is these which determine the behaviour of the stone. For example, if you clean a sandstone containing calcium with acid, it will dissolve.

Igneous Rock

The two types of igneous rock found in the UK are granite and basalt, although only granite has been used extensively as a building material, and then mainly in Scotland and the south-west of England. These are both very hard and whilst they can be chipped, are resistant to damage from frost and careless maintenance. These types are glassy or crystalline. They do not contain fossils. They can have banding which may be mistaken as sediments. Igneous rock can be cleaned with abrasive methods such as sand blasting.

Sedimentary Rock

There are three types of sedimentary rock:

• clastic
• biogenic
• chemical

Clastic rocks are formed from previously eroded rocks. They have a granular texture of pebbles, sands or clays. The largest ‘grains’ are found in rocks such as Hertfordshire Puddingstone. Sandstone has been used widely for buildings and architectural details. The aggregate may be loose, compressed or cemented together with calcite, silica or iron oxide. If a clastic rock is red or brown in colour, this indicates iron (ferruginous). If it effervesces with dilute acid it is calcareous. If it does not effervesce and cannot be scratched, then it is siliceous.

Chemical sedimentary rocks are formed from dissolved minerals. These include limestone, flint and chert.

Biogenic sedimentary rocks are made from the remains of microscopic organisms. The best known example is chalk.

These categories are not firm and combination rocks are common, for example a sandy limestone.

Metamorphic Rock

Metamorphic rocks are igneous or sedimentary rocks which have been changed by heat and/or pressure. The two common rocks used in buildings from this category are slate (from clay) and marble (from limestone). These tend to have the same chemical behaviour as their parent. Thus marble is damaged by acid, like limestone.

Cast Stone

This is manufactured stone. The term 'artificial stone' was commonly used in the 19th century, but later the usual names were 'concrete stone', 'cast stone', or 'cut cast stone'. Cast stone is a mixture of water, sand, coarse aggregate, and cementing agents.

Cast stone gained popularity in the 1860s and its use became more widespread with the development of the Portland cement and concrete industries at the end of the 19th century. However most capitals and other stonework continued to be carved; in London, Bath stone was a common material.

In the early decades of the 20th century, cast stone became widely accepted as a cheap substitute for natural stone. It was sometimes used as the only exterior facing material for a building, but was more often used as trim on a natural stone or brick wall.

In many early 20th century buildings, cast stone was used for exterior window and door surrounds or lintels, copings, parapets and balustrades, banding courses, cornices and friezes, as well as sculptural ornaments.

Coade Stone was one of the best known of these products and was used for architectural ornaments and trim, and occasionally for interior decoration.

Like sandstone, the chemical behaviour of cast stone will reflect the ingredients used; for example acids may damage it. If the stone has split or broken, look for different materials on the surface and inside the stone; the interior will comprise an aggregate and a binding material.