Sedimental Sentiments

Mary Magdalene, Titian

"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

St. John the Baptist 
Charleston, SC

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.

Students learning to carve with Indiana limestone at
The American College of the Building Arts

Alabaster

Alabaster Capital, The Breakers
Newport, RI

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.

Next we'll wrap up the series with the preferred medium of the great masters, metamorphic stone.

Contributed by Patrick Webb