“…with regard to lime we must be careful that it is burned from a stone which, whether soft or hard, is in any case white. After slaking it, mix your mortar, if using pitsand, in the proportions of three parts of sand to one of lime; if using river or sea-sand, mix two parts of sand with one of lime.”
~Vitruvius

The Kheops Pyramid used gypsum mortar in the interior with a lime mortar washed exterior

Mortar: the tie that binds

Mortar was well known two thousand years ago when Vitruvius, the Roman architect, recorded a recipe for it that is quite close to what is done today. The first true mortar to move beyond simple applications of sticky pitch or bitumen tar was made by burning gypsum mined from desert deposits. Gypsum is calcium sulfate, with extra water chemically bound to form a hydrate; the application of fire drives away the water. Finely powdered and mixed with sand, the gypsum mortar will rehydrate when exposed to water, heating up as it binds to the water molecules. This mixture of sand and gypsum is called plaster of Paris today, and it raised the stones of Egypt into the heavens.

The next mortar, and the one that is still most commonly used for masonry, is made by burning limestone. Limestone is primarily composed of calcium carbonate; much like gypsum, it partially decomposes when heated. A carbonate is chemically composed of one carbon and three oxygen atoms, and when calcium carbonate is heated, it gives up one carbon and two oxygen atoms as carbon dioxide gas. The remaining calcium oxide, called quicklime, is slaked with water to form calcium hydroxide. This compound will slowly absorb carbon dioxide from the air, converting back into rocklike calcium carbonate. This artificial rock has become the foundation of nearly everything in modern architecture, tying together stone and brick with equal ability.

Roman brick and flint wall at Burgh Castle, Norfolk

Brickmaking

Bricks were developed first as lumps of dried mud or clay, baked by the sun. Etruscans developed the art of firing clays in higher temperature ovens, which chemically alters the brick material to render it resistant to weather and damage. Under Roman influence, brickmaking became a state industry, widely practiced across the growing Empire. Brickmaking was so widespread that bricks or the materials to make them were only rarely transported further than a few miles from the source to point of use. Bricks were, and are, made in all shapes and sizes, from a variety of materials, and with different purposes in mind. The constants are simple: clay earth, some sand filler, water, and heat.

A typical recipe might include two parts clay to one of sand, ground together with enough water to make a thick plastic paste. The shapes would be formed in rectangular wooden molds, and the bricks left for days or months to age and slowly dry before firing. The aged bricks are baked in wood-fired updraft kilns at temperatures ranging from 650 to 1100C depending on position within the furnace. The firing process results in a variety of permanent chemical changes to the bricks, and can be tailored to produce a range of colors from the same base ingredients. Up to about 600C, water is still being eliminated; even though the boiling point of water is only 100C, there is water chemically bound in the clay that must be removed. Above 600C, the iron oxide maghemite that is present in most clays turns to hematite, giving brick its characteristic color. Around 1000C, minerals like feldspar and amphibole will vitrify and turn to glass, and the clay will decompose and form a variety of silicate compounds with iron and aluminum, resulting in the darkened, glossy look of burned brick. At the extreme upper end of furnace capability, 1200C is sufficient to fire kaolinite clay into stoneware. [1]

Tribes with easy access to clay might be expected to make brick a routine element in their masonry. The Hrothi might prefer rock, but they doubtless understand the art of brickmaking, along with the Janoa, The Waerd, and the Erishe.

Stonemasonry

Stonecutting

Some rocks, like sandstone and some tuffs, are soft enough to saw as if they were wood. Most must be split or hewn from the bedrock to separate blocks or slabs. Firesetting is one way to do so, where a fire is built against the rock and then dashed with water. Well suited to breaking up a vein for mining, this way is usually too imprecise for quarrying.

Scottish quarrymen split granite with the plug-and-feathers (1939)

Plug-and-feathers have been used to split great stones since the Egyptians started building pyramids. First, a line of holes is drilled along the desired split; then, two half-round shims (feathers) of bronze or steel are placed in each hole and a wedge is slowly hammered in. “Give me a lever and a place to stand and I will move the earth”, said Archimedes, and a similar application of mechanical advantage is at work here. The hollow tapping sound of the hammer changes to a solid ringing as the wedge seats itself, followed by the groaning crack of the stone splitting open.

Materials

Sandstone

Sandstone and Greywacke wall, Siccar Point, Scotland

One of the classic sedimentary rocks, sandstone is composed primarily of silica or feldspar sand that has become lithified, or turned to rock by the action of natural mineral cements such as calcite, more silica, or clay (in which case the sandstone is called wacke). Relatively hard and abrasive, yet easy to cut and carve, sandstone is an easy and long lasting building stone as well as an excellent material for grinding and honing tools. In addition to being a good building material, grades of sandstone with fine and uniform grit would be prized as whetstones, and could be an important trade item for tribes such as The Waerd or Owem with natural access to the stone.

Tuff

Tuffs are formed of compacted volcanic ash, thrown out during eruptions and then formed over centuries into deposits of relatively light and soft, yet strong rock. Weathered ashes form the pozzolana that made Roman concrete possible, while similar rocky volcanic ash lithified with mineral cements forms tuffs. An important deposit of this type was in the Roman town of Gabii, just east of Rome, where a well-cemented tuff unusually rich in rock fragments called Lapis Gabinus was quarried. Other tuffs such as Tufo Lionato have a higher proportion of volcanic glassy ash; these are lighter, but less strong. [2]

Limestone and travertine

Limestone and travertine are both sedimentary rocks composed mainly of calcium carbonate. Limestone is found in large deposits all over the world, formed from the cast off shells of tiny, ancient sea creatures. Travertine, although still classified as limestone, is usually formed from hot spring water rich in carbon dioxide, sometimes with the help of calciferous bacteria. Both limestone and travertine are easily cut, quarried, and carved, although travertine is usually a little harder and stronger. The largest commercial deposits of travertine are in Italy, where it has been quarried since antiquity. [2]

Marble

Marble is the metamorphic product of limestone subjected to high pressure, as when tectonic plates fold over themselves and push up into mountains. Marble carves easily and has long been a key material for sculptors and architects. The light and dark grey banding and swirling patterns that we associate with marble stem from thin layers of impurities in the original sedimentary limestone, such as clay, sand, or other minerals. Many other colors are possible, ranging from pure white to black, with pink, yellow, green, and violet all in between.

Granite

Rock for Ancient Egyptian monuments was cut in the Aswan quarries on the Nile. Blocks were hewn out and barged to the pyramids, and fine stonework was also done on site: some unfinished sculptural pieces still lie in quarries, still one with the bedrock. The formation process of granite from liquid magmas typically produces interlocking, intergrown crystals of sodium/potassium and magnesium feldspars. This results in rock that is hard and tough, as well as massive, or lacking the veins or internal structure common in other rocks. Granite is consequentially very difficult to carve, although taking the time to do so will reward the mason with a long-lasting and durable creation. The Hrothi would have masters of granite carving, doubly so considering the frequency of its occurrence in the mountains.

Masonry in Roman Architecture

The Forum of Caesar

Constructed in the Roman Republican era in the 1st century BC, the Forum of Caesar exemplifies the Roman engineering approach to architecture. Lapis Gabinus walls and pillars rise into capstones of travertine, which support arches of additional Lapis Gabinus tuff with travertine capstones. A second building level is constructed entirely of lighter Tufo Lionato. All the structural stone was originally faced with a veneer of marble. The net effect is to utilize the tuff in the stronger compressive dimension, with denser but stronger travertine at key stress concentrations. Less dense tuff on the upper story minimized the total structural load. Of course, no discussion of Roman masonry would be complete without concrete, which I have covered previously. Here, as always, engineering expertise both promotes and enables the vision of the architect. [3]

Masonry in Elyria

Rock is heavy and had to move, and most construction will be done with local stone. A look at feudal Europe reveals very few castles constructed entirely of granite, but many built from sandstone, limestone, and field stones, or even bricks, which will do well enough for protection from arrows and fire. The masons of Elyria will likely follow a similar path, using clever engineering to match the economy of local stone for the majority of walls, stone buildings, and even castles, with granite or other resilient rock imported for areas around the gates and towers where strength and toughness is at a premium. Trade in sculptural elements such as marble could be widespread, providing additional income for stonecutter quarries in Hrothi or Erishe lands just as they did in Roman times.

Further reading

[1] R. Scalenghe, et al, “Material sources of the Roman brick-making industry in the I and II Century A.D. from Regio IX, Regio XI and Alpes Cottiae,” Quaternary International 357[30] (2015) Link

[2] M. Jackson and F. Marra, “Roman Stone Masonry: Volcanic Foundations of the Ancient City,” American Journal of Archaeology 110 (2006) Link

[3] M. D. Jackson, et al, “The Judicious Selection and Preservation of Tuff and Travertine Building Stone in Ancient Rome,” Archaeometry 47[3] (2005) Link

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