Dunhouse: home

The Stone Story

How does the stone you see on the outsides of buildings get there?

This part of the Dunhouse web site is aimed at explaining the stone story, from quarry to shop front.

Stone Story Stone Story Stone Story You will find information about how the stone was formed, how it is quarried, the bedding principles of stone and how it is finally turned into the finished form.

"Stone is a miracle of nature. No two quarries are alike and all stones vary within the same quarry. The correct selection taxes the skill of the quarryman. A quarry is like a book written in a strange language - it must be studied to be understood." Quarryman quoted by A. Clifton-Taylor

Geology

Geologically, sandstones are classed as sedimentary rocks. They are formed by the weathering of metamorphic rocks, granites and other igneous rocks or of other sedimentary rocks. These parent rocks eventually disintegrate into their constituent mineral particles - these particles are known as sediment, and that originating from a granite, for example, will consist of quartz, mica and feldspar.

The particles are transported by the wind and glaciers but most importantly by water which sorts them until they are finally deposited in uniform layers on the floor of seas, rivers and lakes, etc.

The degree and method of sorting determines the texture of the resulting sandstone. In one area there may be coarse, gritty stone whilst in another a fine-grained, even-textured stone. The particles can be transported over large distances and the resulting sandstone may contain sediments from many different sources

Indeed geologists date and trace back to source the particles of sandstones as a means of determining the history of particular regions. Eventually the particles become cemented together, water is lost from the sediment and, together with the effects of enormous pressure, a sedimentary stone is formed.

Quarrying

The method of quarrying sandstones varies greatly - in some quarries where the stone is highly laminated it is lifted off using a crowbar or the bucket attachment on an excavator; this is often the case in quarries supplying paving stone. Other quarries extract stone using 'plug and feathers'.

A row of holes is drilled along the line where the block is required to be split. Two long strips of metal ('feathers') with a long wedge ('plug') between them are inserted into each hole. Then the plugs are hammered home down between the feathers a little at a time in series so that the block splits cleanly.

Perhaps the most common method of quarrying is to use black powder explosive. Again a row of holes is drilled along the block and a small charge of black powder explosive is inserted into each. The charges are all detonated simultaneously, so splitting off blocks of stone from the quarry face.

Indeed the quarrymen are so adept at using the explosive that blocks can be split into manageable sizes at the quarry face for transporting straight into the sawing workshops. However, blocks of stone produced from large-scale blasting operations using dynamite are liable to contain fractures.

Natural forces have dictated that every piece of stone is unique and so poses different problems in extraction. It is this variety that provides the essential beauty of stone.

Bedding Principals

The layered structure of sedimentary rocks must be kept in mind when placing stone in buildings. A bed of stone is rather like a ream of paper, both must be restrained to prevent each layer or page separating and being lost. Thus stone should be placed on its natural bed, i.e. so that each layer is horizontal, just as it was laid down geologically. Edge bedding, where the layers are vertical and perpendicular to the face of the wall, is used for cornices and copings (but not for the corner stones, which should be naturally bedded).

In sedimentary stone the thickness of the beds may range from a few millimetres to over two metres. This variety does not necessarily indicate differences in quality - veining adds to the richness of texture and colour variations in stone from a particular quarry are quite normal.

Processing

Heavy duty equipment is necessary to transport the blocks of stone from the quarry to the processing yard where they are sawed into a series of slabs. Secondary sawing reduces the slabs into dimensional masonry.

If the requirement is for simple ashlar (in other words, straightforward squared and polished panels of stone), the stones will now be complete. More intricate work can be carried out on profiling saws and planning and polishing machines, and of course by hand.

Today it is practical to incorporate within stonecutting machinery hardware which can be programmed to accept instructions from Computer Aided Design systems. Handworking of stone, however, retains an essential place in the last stages of processing, when highly skilled masons work the more detailed stones to their finished form.

Stone is a wonderfully tactile and versatile material, and to watch finished stone emerge in a variety of forms from a piece of rock is an exciting experience. By comparison, the final process of checking and palletising seems rather an anti-climax, but it is nonetheless vital to the organisation of delivery sequences and the safe haulage of material over often considerable distances.