C-
Not the simplest question to answer, but here's the simplest version of an answer I can think of.
Setting aside the question of hydraulic lime - a different species than ordinary lime (and a different argument altogether), the biggest difference is that by and large American lime has a higher, and in most cases much higher, magnesium content (the highest magnesium limestones are termed "dolomitic"). High calcium lime, typically found and used in Europe, requires several months of slaking in putty form to achieve high water retention and plasticity. That explains the European aged lime putty tradition.
Dolomitic lime, with high magnesium content, already has a microstructure that provides the required water retention and plasticity, even as a dry hydrate with no aging. In fact, they knew 170 years ago that dry lime hydrate produced mortar with superior properties to those made with lime putty.
In areas where they used oyster shells or high calcium limestone for lime production in the US, similar practices to those in Europe made some sense. But for the rest of the country, there would be no good reason for aging as putty and there is strong evidence that our tradition was exactly the opposite - they hot-mixed quicklime (unhydrated, unslaked lumps of burnt limestone) with damp sand and then went ahead and used it without aging.
Corps of Engineers specifications called for only two days of slaking, and strictly for the purposes of lump reduction. For the same reasons our $8/bag Home Depot dolomitic Type S mason's lime is every bit as good as the $110/5-gallon bucket lime putty that is being hawked as the definitive choice for historic work. It doesn't need to be putty and it doesn't need to be aged to work perfectly well in mortar.
Still, I support repair and replacement "in kind" as a principal, for both technical and philosophical reasons. That's why the forensic record is so important. For some very limited types of work and for certain sites, lime putty is an historically and technically appropriate material. But that doesn't mean we need to bring it from Europe and it doesn't justify the over-reaching we see by some of the lime putty guys. Too often they are misusing the public forums in which they speak, and they are basically treating well-intentioned architects and consultants like a bunch of suckers.
Mike E
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> Mike, I think I am getting closer. But, to further demonstrate my ignorance, and (I am sure) to mangle the discussion, what is the difference between European lime and American lime (not to mention Honduran lime). Your going to swear at me, but, really, that's the level at which I am (mis)understanding the discussion. c
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