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Hello all:
In the past, dialists have discussed the
techniques, tools, and materials for installing sundials directly into
stone (i.e. a metal or stone vertical wall sundial that is inlaid directly into
a building's outer stone wall or a stained glass sundial that's set in
a window opening as well). The key word here is
"mortars".
I'm in a stained glass discussion group, similar to
our sundial group. Right now we are discussing mortars used to
install stained glass windows in stone walls. I thought some of
you might be interested in the expert opinion and experiences of Mr.
Geoffrey Wallace. I believe his mortar recipe would work equally well for
inlaid stone and metal sundials also.
From: Geoffrey Wallace <[EMAIL PROTECTED]>
Subject: Mortars Date: Sat, 31 Jul 2004 Linda et al, Thanks for raising this very interesting subject, you do help to keep this forum moving along. We use traditional lime putty mortar for bedding our windows as it is tried and true and sympathetic to both the building fabric and the windows. From your description of a mortar with a brittle crust and soft interior I believe we are talking about the same material. To appreciate lime mortar you need to understand what is called the 'lime cycle' which begins with limestone from a quarry [calcium carbonate, CaCO3]. This material is burnt in a lime kiln which drives out carbon dioxide, CO2, leaving behind quick lime [calcium oxide, CaO]. Caution: quicklime is a very dangerous and volatile substance that can cause nasty burns/eye damage. Quicklime is then added to water in a lime pit which creates a violent reaction involving heat, bubbling and spitting [eye and skin protection should be worn]. The quicklime draws in hydrogen from the water creating lime putty [calcium hydroxide, Ca(OH)2. Lime putty is combined with sand to make basic, traditional lime mortar. After the lime mortar is trowled into the building it begins to dry, driving out water, H2O. As it dries the exposed surface of the mortar attracts carbon dioxide, CO2, from the air and becomes, once again, calcium carbonate, CaCO3. Below the surface, where there has been no contact with the air, the mortar remains soft and putty like. That is the basic recipe for lime mortar but you also asked why we choose this material. Well, we are currently working on the conservation of windows at St. Paul's Cathedral, Melbourne and have just completed the west end. The west windows [Clayton and Bell] were installed about 1890 and the glaziers were just as enthusiastic about the new wonder product, portland cement, as was Thomas Hardy. They knocked up a rich mix. Over the last 110 years, due to failing footings, the north wall of the cathedral has walked 30cm westward. As it did it stretched the west wall, including the windows, by 30cm. The result of structural movement combined with rich cement mortar was vertical splitting of five large windows to a drastic degree. If the windows had been bedded in lime putty mortar they would have pulled free from the glazing channel and remained intact. While I am at it I will try to clarify a few other terms that seem to have been confused: Hydraulic lime is lime that will set quickly in wet conditions, possibly even under water. It has had powdered, fired clay added to the quicklime during slaking and is the same thing as potsalanic lime. It sets up hard and brittle all the way through and is not recommended for bedding windows. Hydrated lime is quicklime that has been lightly sprayed with hot water to reduce its volatility. This is the white powder that you buy at gardening shops to apply to your garden. Although the purists disagree, hydrated lime will make a quite adequate lime putty for mortar, but quicklime is best. Waterproof lime putty is created by adding either shaved tallow, linseed oil or skim milk at the most volatile point of slaking when the generated heat will instantly and permanently disperse it throughout the lime putty. This would be suitable for the west coast of Scotland and similar climates but is hardly required anywhere else. Keep in mind 'waterproof' is not necessarily a good thing in old buildings that were designed to breathe. Lime putty should be left to sit, covered with water, for a minimum of 3 months before use [the Romans recommended 2 years]. The longer it sits the more 'buttery' the putty becomes and well matured lime putty is a true joy to use as the resulting mortar will behave almost like cream cheese and remain attached even to an upturned trowel. It's a lot more fun than any other bedding material. Cheers, Geoffrey P.S. Up to a certain degree lime putty mortar is also self repairing. If, for whatever reason, the cured mortar is cracked then the next time it gets rained on the exposed mortar will carbonate creating expansion and filling the cracks. |
