Building Stone

Building stone and clays have been used by builders since man first realized that he could construct safe, durable and long lasting structures using stone and timbers. Structures made from logs and hewn timbers were the first fortified homesites. However, timber structures didn't weather long or last long in battle. All wooden fortified structures were eventually replaced with timber and stone ones. Timber framed structures within outer stone curtain walls were the first long lasting castles. As builders working with large stone building became more skilled, castles became almost all stone. Timbers were still used for roof and interior support strucures. But, stone became an important building material and men skilled in working with stone and mortar became known as Masons. The skill, art and profession of Masonry was born.
And where enough suitable building stone did not exist man soon learned that clays, when properly selected, prepared and used, made excellent structures also. Building bricks, Adobe, and Mud and Wattle are the most common uses of clays in building.
In the following series of pages we will attempt to describe the more commonly used rocks and stones used for building. Our emphasis will be on those types and classifications of building stone that may be used to construct castles. And we will also cover the many uses of clay in the castle construction process.
The primary groups of stone which we are concerned with are as follows.

Sedimentary Rocks: Limestone and Sandstone

Metamorphic Rocks: Slate and Marble

Igneous Rock: Granite

Miscellanous: Field and river rock


What is limestone?

Limestone is a sedimentary rock consisting primarily of lime which was formed by a variety of biological, chemical and physical processes that took place over millions of years. Generally most limestone is chiefly composed of calcium carbonate, the organic remains of shells and coral.
Dolomitic limestones are chemically magnesium carbonate. The fossils and other sedimentary structures which limestone contains provides a key as to the environment in which limestones where formed.
Limestone was formed from materials that were deposted as soft sediments laid down as layers in beds under water. Over millions of years these sediments were buried, mineralized, compressed and became cemented together into a rock.
Those limestones with a porosity of less than 5% and will take a full polish are called hard limestone.
Limestone used outside must be protected from water and frost damage. If the porosity of the stone is to high, water may collect in the stone, freeze, and then swell. This may cause the stone to flake, crack or to split.
Limestone is also used to make lime cement. This process involves burning the limestone in kilms to produce the lime cement. And this lime cement when mixed with sand and water would be used to cement the building stones or fired clay bricks together to form walls and other parts of the castle. This process of making and using lime cement will be discused in detail later.

What is sandstone?

Sandstone is a sedimentary rock consisting primarily of quartz, occasionally felspar and other sand particles, and small rock particles all cemented together into a firm stone. These materials eroded from older rocks were sorted and deposited by rain and wind to form beds of layered materials. The sandstone was then formed by a chemical and physical process similar to the formation of limestone. The various minerals, such as iron oxides and carbonates, that leached into the layered sand beds gives sandstone its rich and varied colors.
Sandstone is one of our most plentiful, widely distrubuted, and useful building stones. Many fine public building are constructed with various grades of construction quality sandstone. Some grades of sandstone are also rather easy to carve into decorative forms. Depending upon the hardness and texture, sandstone slabs can be carved into a variety of icons, decorations and forms to enhance the walls, doorways and gateways of castles and other buildings.

What is Slate?

Slate is a metamorphic rock. This means that this fine grained rock is the result of the actions of pressure, heat and water upon clay and silt sediments deposited on ancient sea beds over millions of years ago. Mountain building forces subsequently gradually folded, crumpled and compressed these deposits into the shale. This has resulted in a very compact and highly crystalline rock structure. At the same time the resulting intense heat and pressure changed the orifional clay and silt particles into new minerals such as mica, chlorite, and quartz. Slates vary in composition, structure, color, and durability because of the degree to which their mineral content has been altered and which is neither uniform nor consistant.
The metamorphic process of geologic change necessary to produce slate are dependent upon large scale movements in the earth's crest and the intense heat and extreme pressure generated therein. For this reason, slate is found only in certain mountainous regions of the earth. And only a few areas of these regions have quarries that produce slate suitable for making slate stone for building purposes.
There are three general classifications of slate: mica, clay and igneous. For our discussion we will deal with the mice form. This is because this slate is what slate roofs are made from. Mica is a generic term for any group of minerals that crystallizes in thin easily separated layers. In slate, the mineral is primarily silcon dioxide in the form of crystalline quartz.

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