Stone Queries - July/Aug 2002

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Created: Monday, 01 July 2002 21:32

Q: What is quarry sap?In the book, The Agony and the Ecstasy, Michelangelo is said to have draped the David marble in damp cloth after every carving session. What was he doing?

A: While I wouldn’t begin to question Michelangelo’s approach, I strongly suspect that description is more artistic license of the author than artistic technique of the sculptor.  Damp cloth draping is essential when modeling in clay but as far as I can determine of little or no value in marble.

All stone contains varying amounts of moisture, or pore water, when freshly quarried.  In English-speaking countries this moisture is usually called “quarry sap” and almost always stoneworkers claim that freshly quarried stone carves more easily than stone that has dried out.  Often it is further claimed that once a stone has dried out, even immersing it in water will not restore it to its original easily-carved condition.  The marble block used for David was quarried, partially carved, then abandoned for many years before Michelangelo started carving it.

I have carved some limestones and sandstones that soften when wetted, but these have a porosity of up to 30% and contain clay as well. Marble typically contains less than 1% moisture by weight, and many marbles from the Carrara area contain less than 0.1%.  As a block of marble dries out, the calcium carbonate in the pore water would precipitate along the crystal boundaries, but would that be enough to affect carving or prevent later water penetration?  And yet something can happen.  I have seen a block of statuario marble that changed from solid stone to a sugary, crumbly mass after sitting outside a Pietrasanta studio for a year.  I am still looking for any scientific studies that confirm the lore or document precisely the physical and chemical changes occurring with loss of pore water that would affect carving.

I have been warned to avoid unscrupulous stone dealers selling old dry blocks to sculptors who don’t know any better.  But how quickly does a block dry out?  Not many of us have the luxury of plucking a block from the quarry face as soon as it’s extracted, so I have stopped worrying about stone freshness.

So what was Michelangelo doing?  Was he even doing it?  The only contemporary reference I have been able to check so far is Vasari’s, The Lives of the Artists and he does not mention damp cloth draping.  Or did Michelangelo know something about stone that I don’t?  You bet he did!  But was this it?

Feel free to contact me: Ron Geitgey, (503) 235-3474, This email address is being protected from spambots. You need JavaScript enabled to view it..

Stone Queries - May/June 2002

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Created: Wednesday, 01 May 2002 20:12

Q: I would like to carve some pieces to place in my garden.  What types of stone can be placed outside?

A: Depending on your garden and your expectations, nearly every sculptural stone can be placed outside.  But note the qualifications at the beginning of that sentence.  The reaction of various stone to the environment can include dissolution, loss of high polish and fine detail, bleaching by sunlight, iron staining (rust), algae growth, staining by waterborne chemicals, and disintegration caused by crystallization and expansion of salts and ice within the stone.

Moisture is the single most detrimental factor, whether from rainwater which is naturally slightly acidic, soil moisture wicking upward from the ground, or sprinkling systems repeatedly dowsing the sculpture.

Alabaster (gypsum) and anhydrite are soluble but they can last longer than you may think.   I have had an alabaster piece exposed outside in Portland weather for about three years with very little effect. (Dissolution does suggest artistic possibilities, however.  I recall some fascinating transformations as ice sculptures slowly melted at Camp Brotherhood a few years ago.)  Marble and limestone will dissolve so slowly that the only effect you may see in your lifetime is dulling of a high polish.

Soapstone, although soft, is very stable chemically.  It has been used for centuries as a durable building stone throughout Scandinavia, but it may contain pyrite, which can cause iron staining.

Stone porosity, including holes in travertine, spaces between grains in sedimentary rocks (e.g., sandstone, limestone), or fractures in any rock type, can be an important factor.  If a stone can soak up moisture it can also soak up gardening and lawn care chemicals resulting in white coatings on the sculpture surface and possibly providing additional nutrients for algae.  But then again, what are your expectations?  I have a piece carved of porous volcanic tuff, sitting directly on the ground.  In the dry summer it’s bright yellow and red, but lush green all winter when it’s wet.  It’s a sculpture for all seasons.

Granite and basalt are very durable although some contain reactive iron-bearing minerals.  Many sandstones, limestones, and marbles are good choices, especially if they won’t be water saturated.  Avoid black marble and limestone since they quickly bleach or develop white coatings.

If you want to minimize the effects of wicking, isolate the sculpture from soil moisture using concrete or stone pads, bases, or pedestals.  Finally, there are numerous products available for sealing or impregnating stone to minimize water absorption.  I have only limited experience with these so here is an invitation to someone more knowledgeable than I to write a column. Feel free to contact me: Ron Geitgey, (503) 235-3474, This email address is being protected from spambots. You need JavaScript enabled to view it..