medieval-religion: Scholarly discussions of medieval religion and culture
The chemistry of medieval stone artifacts and painted glass
Sculpture, stone construction and glass, have a common problem - carbon
dioxide.
Carbon dioxide is always in the air, and reacts with water to form carbonic
acid. Carbonic acid, while a weak acid, that is it is not highly ionized,
does react with any form of calcium carbonate to form a soluble calcium
bicarbonate. When you consider that this means that limestone, chalk,
marble, coral, and the mortar that is used between building stones are all
rendered more or less soluble by carbonic acid, you can imagine the effect
upon buildings of stone construction more than a hundred years old.
More recently, sulfur oxides in the air from the combustion of coal and
other fossil fuels are the cause of degradation of all of the construction
materials mentioned above. However the solution of sulfur dioxide produces
an acid 100,000 stronger than carbonic acid. The damage that sulfur dioxide
causes in a given period of time is far greater that that done by carbon
dioxide.
In the 1930's, Russell Hall at Teachers College Columbia, was found to have
severe damage to the mortar between the building stones. The damaged mortar
was chiseled out, and replaced by lead.
The lead came in the form of a sort of wool like fiber. Strands were pushed
into the cavity with the fingers, and then a blunt chisel and hammer was
used to pack the lead in. Since lead is very soft and malleable, this formed
a barrier to any contact between the mortar and carbonic acid. Since this
technique would be available in medieval times, it would be of interest to
know of any examples.
Glass is different from any of the materials mentioned to date. In the first
place it is not a solid - technically it is a liquid because it does not
have a crystalline structure. It consists of a complex mixture of calcium
and sodium silicates.
An Internet search for glass, history brought up the following item -
650 BC - First Glassmaking handbook, Assyrian Assurbanipal's Library
Glass is soluble in water. It is so soluble that in some aspects of analysis
of aqueous solutions, the samples to be analyzed must be kept in plastic.
True, it is only soluble in small amounts, but over time, carbonic acid will
damage glass. Carbonic acid is a thousand times stronger as an acid that
silicic acid. Silicates, over time, are converted to soluble bicarbonates,
pitting the surface of the ornamental glass. This damage is much slower that
than done to the several forms of calcium carbonate mentioned above.
I hope that this clarifies some of the discussion on painted glass.
Regards, Jim
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