A GUIDE TO THE CREATION OF STONE
The purpose of this article is to give a brief and simple explanation about how stone, marble, granite and slate are formed and how the different way that each type of stone was formed affects the usefulness of the stone as a potential house sign material.
Stone, Marble, Granite and Slate are all basically a type or class of naturally occurring rock. There are three basic types of natural rock formation.
(a) Igneous – rock created as a result of volcanic activity.
(b) Sedimentary – Stone formed by the laying down and fossilisation of sea or lake bed sediments.
(c) Metamorphic – rock was originally sedimentary or igneous rock that changed (or metamorphosed) over aeons of time. Usually as a result of great heat and / or pressure.
Within each of these basic groups there can be a wide variation of sub-classes of stone each with a different character and nature that will either make these stones suitable for house signs or not.
Igneous rocks are created by volcanic activity are among the oldest rocks on earth. As house sign makers the rock that we are most interested in, within this group is granite.
Granites are some of the hardest and most enduring of the stones worked by man. Granites are formed from superheated volcanic lava, cooled slowly in the air or fast under the ocean and then exposed to enormous geological pressures.
Think of the volcanic activity as a vast super hot food blender, the different minerals that make up the granite are broken down and mixed evenly throughout the resulting material. This violent churning and tempestuous history creates a rock that is incredibly tight grained. The colour of the granite is dictated by the mineral content of the original lava.
The mixed composition of the granite means that generally there is usually only a small variation in colour or pattern compared for instance to marble.
The hardness and resistance to erosion and acidity are the the result of the granite being formed from microscopic plates of silica. Think of glass, think how hard and resistant to wear glass is. Well, granite is super-glass, microscopic glass slivers bonded by heat and layered for strength. It is wonderful to think, that when we are looking at a piece of polished granite we are seeing millions of years into the past, we are getting a glimpse at the very dawn of time!
Sedimentary stone is formed by the laying down and fossilisation of sea or lake bed sediments. Over the years layer after layer of sand, sediment and decaying organisms settle on the sea floor. Each layer captures traces of the environment of the time. Including mineral traces which dictate the colour of the final stone, dust and soot particles in the air and sediment washed down from the land. Even the shells and the hard parts of the bodies of marine organisms living in the sediment at the time time can make a significant contribution to the make-up of the stone.
This is why many sedimentary stones but not all, have fossilised seashells embedded in the material. These layed down sediments have been crushed and compressed by millions of years of geological activity.
The qualities of each type of stone will be dictated by this history. A stone that has its origins at a time when the surrounding seas were teeming with life will be full of shell and fossils. A stone laid down in a lifeless sea will consist only of sand or fine silt.
Sedimentary stones fall into two main sub groups: Sandstone and Limestone.
Limestone is composed largely of the mineral calcite. If the original silt layer was densely populated by marine organisms the calcium in the shells and bones trapped in the leaches into the surrounding silt and forms a limestone. It is important to understand the nature of limestone as it relates to house signs.
Limestones generally have highly visible fossil remains embedded in the stone. These fossils are referred to by masons by the collective term “shell”.
These shells are harder than the surrounding stone, this can make engraving the house sign difficult and perhaps uneven. Limestones are generally white, cream or pale grey in colour . The shell will affect the finished appearance of the stone showing up as darker flecks in the face of the stone.
The big “elephant in the limestone room” as regards house signs is that limestone can be very susceptible to long term water erosion. As the house sign ages the surface will weather unevenly, the shell being harder will not erode so quickly.
Let me at once put this into proportion. We are talking about the natural process of weathering over many decades. Some of our greatest historic buildings are built from limestone and they have lasted for centuries. And lastly, if, like me, you love old stonework you will know that this weathered look is truly beautiful.
British limestones include Portland stone, Cotswold, Bath-stone and Purbeck.
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains. Some sandstones can be very resistant to weathering, yet are often relatively soft and easy to work. It’s a paradox that works to the advantage of house sign makers. Fine grained sandstones will usually engrave superbly. It is however, well to note that coarser grained sandstones, those with a larger grit size, will not always be suitable for engraving and will engrave with less definition or sharpness.
The qualities inherent in the stone will not only vary from stone to stone but also from layer to layer within the same quarry! York stone can be formed with very coarse grain size or be fine grained and engrave very well indeed. Also not all sandstones are soft – York stone can be a very hard stone. Indian sandstones in particular are extremely hard, made from fine silt and very close grained. These stones are very resistant to weathering and wear and will engrave with very fine detail.
British sandstones include York stone, Bargate stone, Derby stone and Red sandstone.
Metamorphic rock is formed from either sedimentary or igneous rock that has been changed by geological activity into a (sometimes) very different form of rock. The two classes of rock in this group that will mainly be of interest to house sign makers is slate and marble.
Slate is a rock formed originally from shale, crushed twisted and compressed by massive forces to create a very fine grained material.
Slate is usually Dark grey, green or sometimes purple.
It is worth noting that it is very rare to find fossils within slate. The original material that went in to making the slate has been so crushed and distorted that any traces of fossil have been destroyed.
One of the defining features of slate is that the Slate is layered in thin beds or strata. These strata are not laid down like sedimentary layers but formed by the stresses imposed by geological pressures and convolutions. Using a hammer and chisel the slate can be split along these strata to create thinner sheets.
The face of the slate resulting from this splitting is called “riven” and is considered by many to be the most attractive feature of slate. This laminated nature gives slate a high structural strength allowing the manufacture of thin house signs and very hard wearing commemorative plaques. The fineness of the grain means that slate will engrave beautifully.
Great British slates include Welsh grey slate, Cumberland green slate and Cornish Delabole slate.
Marble is a Metamorphic sedimentary limestone. Marble is a sedimentary stone that has been changed and made harder by geological movement and pressure. The movement and distortion of the original stone creates the washy marbling patterns so typical of marble. The extra hardness allows the marble to be polished to a high gloss surface. Therefore this is the simple definition of what is a marble is: “Marble is a limestone that will take a polish.”
As marble is created from Limestone it therefore follows that some marbles may be susceptible to water erosion. Not all though, some types of marble are less likely to suffer. White Carrara marble for instance has been used for generations as a long lasting headstone material.
One thing seems clear, when buying or ordering a new house sign or name plaque for your home, take the advice of your friendly stonemason or house sign maker. He or she will have the experience and depth of knowledge to guide you in your choice of stone, granite or slate.