Igneous (Primary)
The Gods of Rock: Pluto and Vulcan
Human
beings have been working stone for a long time. It seems as though
physically our hands and bodies are perfectly adapted for the task.
Likewise, the ability to conceptually form stone into tools, hunting
implements and structures might well be hardwired into our DNA. Such an
enduring material, much of what is gleaned of early civilisation in the
field of anthropology has quite literally been recorded in stone. This
post initiates a three part series to shed a bit of light on the types
of stones that generations of masons and carvers have found most
useful.
We Have Ignition!
We'll
begin with what are called primary rocks*, so classified because they
are the direct result of the original consolidation of minerals into
stone. The common name for this group of stone is Igneous, derived from
the Latin "Ignis" meaning "fire". That name is a
strong indication of the origin of Igneous rocks, deep under the earth's
surface in the lower crust or even the mantle. Oxidized silicon
compounds (silicas, SiO2 or silicates, SiO4) under
extreme heat and pressure form and mix into a liquid, molten state
known as magma. How that molten magma cools distinguishes the two major
sub-classifications of igneous rock.
Pluto - God of the Underworld...
and Riches
Most
folks know Pluto as the counterpart of the Greek Hades, the Roman god
of the underworld. However, the Romans had a more positive view of Pluto
also considering him the god of wealth, a giver of abundant riches.
This was in no small measure due to the mineral wealth the Romans were
particularly adept at unearthing from mining operations.
Plutonic
rocks are Igneous rocks that have specifically cooled, underground
within the earth's crust, surrounded by other rocks. The other rocks act
as a kind of insulation. As a result Plutonic rocks cool very slowly.
This allows time for large and long crystal formation, resulting in a
coarser textured stone. Plutonic rocks are also classified as Intrusive rocks because the magma typically "intrudes" or fills horizontal or vertical strata known as "sills" and "dikes"
respectively. Larger cooled magma chambers can produce "bathotliths",
huge massifs that can account for entire mountain ranges.
The
most common form of Plutonic stone used for masonry is granite.
Although coarse, brittle and hard to work, granite is a beautiful stone
that is exceedingly durable. Because it is hard to achieve fine detail
it is best reserved for large scale, monumental work when carved.
Vulcan - God of Fire...
and Volcanoes
What
is not commonly known is that Vulcan was also the god of metalworking
specifically and crafts in general. Most often in ancient sculpture
you'll find him hammer in hand surrounded by billowing fire and smoke.
The stone temples of the gods were attributed to the hands of Vulcan.
Volcanic
rocks are also Igneous rocks; however, volcanic rocks are formed from
lava, magma that has been discharged from underground into the sea or in
the open air. Therefore, volcanic rocks are classified as Extrusive
rocks as they are "extruded" as lava from underground magma tubes onto
the surface where they rapidly cool. There are many varieties of useful
volcanic rock for masonry and carving. Basalt in particular shares the
density and durability of granite. Yet, because of the rapid cooling,
the crystalline structure basalt is much smaller and finer allowing for
greater detail in carving. The Giant's Causeway on the coast of Ireland
is an amazing example of a volcanic basalt formation popularized by rock gods Led Zeppelin on the iconic cover of their Houses of the Holy album.
Next in the series we'll consider my personal favourite for carving, sedimentary stone.
*The
terms "rock" and "stone" are used interchangeably in this post. Some
consider "rock" to denote the natural form whereas "stone" is
distinguished by or set aside for human alteration.
Sedimentary (Secondary)
Sedimental Sentiments
"Do not keep the alabaster boxes of your love and
tenderness sealed up until your friends are dead. Fill their lives with
sweetness, speak cheering words while their ears can hear, and while
their hearts can be thrilled and made happier by them." - William Congreve
What a great time to be a stone carver. 3 billion years ago would not have done at all. Hell on earth with nothing but pumice, granite and basalt about. If igneous rocks are the spawn of heat and flame, sedimentary stones are the progeny of water and ice. Life, erosion and lots and lots of time has mellowed, softened the waking nightmare into a dreamy paradise.
Sedimentary stones are classified as secondary, that is to say they are the result of a subsequent action. The name 'sediment' derives from the Latin 'sedere' meaning 'to sit down' or 'deposit'. So it is that sedimentary stones form gently, layer after layer, year after year slowly accreting into laminar beds. Let's consider a few common sedimentary stones, how they form and their most appreciated properties.
Sandstone
Sandstone generally is a result of erosion,
specifically glacial action. The ice presses forward slowly,
methodically grinding the mountains of quartz, granite and feldspar,
pulverizing them into sand, abandoned as they recede. Layers of sand
build up, compressing under their own weight. Saturation with water is
the key to turning the sand into stone. Minerals precipitate out of the
water into the tiny spaces between the sand forming a natural cement. If
the deposits are argillaceous, clayey the stone will remain relatively
soft yet easy to carve. Calcareous deposits will produce a sandstone
similar to limestone. Siliceous or silica infiltration can result in the
hardest, most durable of sedimentary stones.
Limestone
The formation of limestone is typically an altogether different process of deposition. Over millions of years marine organisms small and large extract calcium from seawater to form there skeletons. As they perish, their remains collect on the ocean floor. Usually you can see 'bedding', the layers that have compacted and cemented together as clearly as rings on a tree. When cutting and laying sedimentary stone it very important to take into consideration the bedding. The stone is most vulnerable to erosion and frost if the laminar or horizontal face of the stone is exposed, particularly in a vertical orientation. Many limestones are relatively easy to work and can achieve a good level of detail making them an ideal material when learning to carve.
Alabaster
The most common way for gypsum to form is through the process of evaporation.
As water becomes trapped in inland seas, salt levels increase
exponentially. Under the right conditions, particularly when the water
has high calcium content, gypsum will precipitate in large quantities
that accumulate into giant 'massifs'. As these beds compact they
increasingly crystallize eventually forming the highly translucent and
beautiful alabaster. Alabaster can be pure white although it is not
uncommon to find a 'bloody' iron infiltration. Alabaster is a softer
mineral than sandstone or limestone, is easily carved and can be brought
from a lustrous to high polish.
Metamorphic (Tertiary)
Change You Can Believe In
“What you leave behind is not what is engraved on stone monuments, but what is woven into the lives of others.” ― Pericles
Metamorphosis: the changing of form of one thing into that of another. Bernini effectively captures the moment of metamorphosis of the nymph Daphne. Her pleas answered, she is transformed into a tree, escaping the lustful pursuit of the god Apollo. Yet Bernini's work too is a metamorphosis: that of a block of lifeless marble transformed into a frozen moment of movement and emotion. Underlying all of this is yet another transformation, that of the marble itself, belonging to a third or tertiary group of rock classified as Metamorphic.
Marble
Marble starts off its existence as a sedimentary rock, limestone. Through heat and pressure within the earth's crust its molecules are loosed from their bonds. Subject to magnetic forces they reorient, align becoming more crystalline. If the limestone is pure the marble will be an unadultered white such as the Luna marble used for Benini's sculpture. More often the limestone has impurities of sand, clay or other minerals that can provide interesting patterns of veining as well as a large variety of colours, many quite vibrant.
Slate
Under certain conditions clays composed of quartz and
mica will settle in sedimentary beds that later undergo compaction.
These form leaves of shale, a very fissible or flaky stone not typically
useful as a building material. Under given conditions of heat and
directional pressure shale will transform into slate. One of slate's
most useful qualities is its impermeability of water absorption. This
makes it ideal for water tables and as a roofing material. Although most
slate occurs in darker colors there is still considerable variety that
allow for intricate pattern making. As a result slate roofs can be quite
beautiful, require minimal maintenance and often last hundreds of
years. Despite its hardness and density, slate is very finely textured
which makes it a desirable, enduring material for plaques and
gravestones.
Quartzite
As limestone will transform into marble, sandstone subjected to similar conditions will change into quartzite. It completely loses its original sedimentary character ending up with a very homogeneous texture. The resulting stone is typically extremely hard and likewise phenomenally durable. Quartzite is often used for paving of roads because of its character. The ancient Egyptians used quartzite for colossal monumental work. Many of their original statues have endured the elements in good condition for thousands of years.
Schist and Gneiss
Unlike the aforementioned, schist and gneiss are classifications of rocks that have undergone a geological process independent of their chemistry. They are more often than not a fusion of several different types of rock that have become subject to extreme tectonic forces, such as those occurring at continental plate boundaries. These forces causes the minerals in the rocks to separate and begin to form independent layers. Schists have varying degrees of hardness but are characterized by 'foliation' referring to fine layers that can be easily split. Gneiss by contrast is not easily split at the coarser layer boundaries and tends to be a much harder material. Gneiss is a very common material making up a large percentage of the bedrock of the earth's crust.
That concludes this brief series on the three main classifications of rock: Igneous, Sedimentary and Metamorphic. I hope you've enjoyed them! Meanwhile, it's time for me to get back to carving. Look forward to updating you on my next project, a Jacobean fireplace in limestone.
Contributed by Patrick Webb
Sedimentary (Secondary)
Sedimental Sentiments
 |
| Mary Magdalene, Titian |
What a great time to be a stone carver. 3 billion years ago would not have done at all. Hell on earth with nothing but pumice, granite and basalt about. If igneous rocks are the spawn of heat and flame, sedimentary stones are the progeny of water and ice. Life, erosion and lots and lots of time has mellowed, softened the waking nightmare into a dreamy paradise.
Sedimentary stones are classified as secondary, that is to say they are the result of a subsequent action. The name 'sediment' derives from the Latin 'sedere' meaning 'to sit down' or 'deposit'. So it is that sedimentary stones form gently, layer after layer, year after year slowly accreting into laminar beds. Let's consider a few common sedimentary stones, how they form and their most appreciated properties.
Sandstone
 |
| St. John the Baptist Charleston, SC |
Limestone
The formation of limestone is typically an altogether different process of deposition. Over millions of years marine organisms small and large extract calcium from seawater to form there skeletons. As they perish, their remains collect on the ocean floor. Usually you can see 'bedding', the layers that have compacted and cemented together as clearly as rings on a tree. When cutting and laying sedimentary stone it very important to take into consideration the bedding. The stone is most vulnerable to erosion and frost if the laminar or horizontal face of the stone is exposed, particularly in a vertical orientation. Many limestones are relatively easy to work and can achieve a good level of detail making them an ideal material when learning to carve.
 |
| Students learning to carve with Indiana limestone at The American College of the Building Arts |
Alabaster
 |
| Alabaster Capital, The Breakers Newport, RI |
Metamorphic (Tertiary)
Change You Can Believe In
 |
| Apollo and Daphne, Bernini |
Metamorphosis: the changing of form of one thing into that of another. Bernini effectively captures the moment of metamorphosis of the nymph Daphne. Her pleas answered, she is transformed into a tree, escaping the lustful pursuit of the god Apollo. Yet Bernini's work too is a metamorphosis: that of a block of lifeless marble transformed into a frozen moment of movement and emotion. Underlying all of this is yet another transformation, that of the marble itself, belonging to a third or tertiary group of rock classified as Metamorphic.
Marble
Marble starts off its existence as a sedimentary rock, limestone. Through heat and pressure within the earth's crust its molecules are loosed from their bonds. Subject to magnetic forces they reorient, align becoming more crystalline. If the limestone is pure the marble will be an unadultered white such as the Luna marble used for Benini's sculpture. More often the limestone has impurities of sand, clay or other minerals that can provide interesting patterns of veining as well as a large variety of colours, many quite vibrant.
 |
| Rossa Verona marble, Basilica di Santa Maria Maggiore a Bergamo |
 |
| Charleston, SC |
Quartzite
 |
| Amenhotep III, Thebes |
As limestone will transform into marble, sandstone subjected to similar conditions will change into quartzite. It completely loses its original sedimentary character ending up with a very homogeneous texture. The resulting stone is typically extremely hard and likewise phenomenally durable. Quartzite is often used for paving of roads because of its character. The ancient Egyptians used quartzite for colossal monumental work. Many of their original statues have endured the elements in good condition for thousands of years.
Schist and Gneiss
Unlike the aforementioned, schist and gneiss are classifications of rocks that have undergone a geological process independent of their chemistry. They are more often than not a fusion of several different types of rock that have become subject to extreme tectonic forces, such as those occurring at continental plate boundaries. These forces causes the minerals in the rocks to separate and begin to form independent layers. Schists have varying degrees of hardness but are characterized by 'foliation' referring to fine layers that can be easily split. Gneiss by contrast is not easily split at the coarser layer boundaries and tends to be a much harder material. Gneiss is a very common material making up a large percentage of the bedrock of the earth's crust.
 |
| Gal Viharaya Buddha |
That concludes this brief series on the three main classifications of rock: Igneous, Sedimentary and Metamorphic. I hope you've enjoyed them! Meanwhile, it's time for me to get back to carving. Look forward to updating you on my next project, a Jacobean fireplace in limestone.
Contributed by Patrick Webb
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