So what is the "hydrate"? In the lime cycle, it is the product of
slaking: calcium hydroxide. The important point to remember is that the hydrate can be
either dry or wet, depending on how much water is used for slaking. If only just enough
water is used, the hydrate will be dry, but if an excess of water is used, the
hydrate will be a putty - or, with the addition of even more water, limewash.
Returning to the mid-1970s, at that time most of the written advice about lime
came from government or commercial sources and it dealt almost entirely with the use of
hydrated lime -- the dry powder bought in bags - as a constituent of cement-lime-sand
mortars, A favourite (and typical) publication of the time was Lime in Building, by
the charmingly named British Quarrying and Slag Federation; all its recommendations were
for cement-lime-sand mixes, and it says: "Lime putty is rarely made nowadays on site
from quicklime but is generally produced from hydrated lime. This is done by the gradual
addition of hydrated lime to water ... to form a paste". However, various individuals
were beginning to research lime in depth and rediscovering techniques for slaking lime
from quicklime. For instance, a contemporary of mine at the Architectural Association,
Mike Wingate, wrote a thesis on lime and went on to be a leading contributor to the field,
including writing a book Small Scale Lime Burning (1985).
Another leading light was John Ashurst. His expertise and practical experience
in the Ancient Monuments and Historic Buildings Directorate made him very influential, and
through his teaching and writing he produced the classic Mortars, Plasters and Renders
in Conservation (1983), and then in 1988 the magisterial English Heritage Technical
Handbooks Practical Building Conservation. Here at last was the advice we needed.
We were to forget about bagged hydrated lime and go back to basics, buying quicklime and
slaking it. "Summary of procedures to obtain optimum performance from mortars based
on non-hydraulic lime. 1. Slake freshly burnt lime on site with enough water to
obtain a soft mass of putty. Continue stirring during the slaking process. Sieve to remove
lumps. Keep the putty under a water layer for at least one week to ensure thorough
slaking".
Inspired by this, we persuaded John Ashurst to come to the Museum to give us a
"lime day" in January 1985, to try to improve our techniques. We had been using
lime-only mortars (no cement) for 10 years, mainly with bagged hydrated lime, but not with
complete success. Gradually our techniques improved. We built lime pits for slaking, and
by the time we built Longport Farmhouse in 1994 we were confident in slaking and using
lime. The mortar in Longport has certainly worked extremely well.
But then, two years ago, Gerard Lynch dropped a small bombshell into the cosy
world of lime putty, in the form of a paper about lime mortars in the Journal of
Architectural Conservation (March and July 1998). Gerard had served an apprenticeship as a
bricklayer and became a master of his craft. He was one of the few people to know, and
more importantly, to care about, the secrets of gauged brickwork - that is, brickwork
using bricks that are cut and rubbed to very precise dimensions and laid with very thin
joints. In 1990 he published his first book, Gauged Brickwork, then in 1994 Brickwork
: history, technology and practice which established his reputation as one of the
leading historians of architectural brickwork. I first met him in 1991 when I asked his
advice on the Museum’s exhibition on Bricks and Brickwork (in the Petersfield brick
drying shed). In 1993 he started running courses at the Museum and is now one of our most
popular tutors.
In his paper Gerard argued that the conservation world’s focus on lime
putty had made us forget the fact that historically most mortar for brickwork had been
prepared in a quite different way. One of the most telling parts of his argument was that
using lime pits for putty for mortar would simply not have been able to keep up with
demand on busy sites. Twenty Georgian bricklayers building a terrace of houses, for
instance, would need about 7 tons of slaked lime every day, which would require huge pits,
worked on a rota. Lime certainly was slaked to putty in pits, but the product, the finest
slaked lime available, was used mainly by plasterers.
So how was the mortar for general brickwork made? The answer lay in
Gerard’s own experience as an apprentice, when he learned the techniques. The
quicklime would arrive fresh and still warm from the kilns - or even be burnt on site - in
the form of stones, which were broken up as small as possible. A "banker", or
mixing platform, was formed from close-set wooden planks, and on it was placed a large
ring of sand, perhaps a yard across. The quicklime (called "stone lime") was put
in the middle of the sand and lightly watered, which would start it immediately slaking,
the stone lumps beginning to spit, pop and crumble. Immediately, the ring of sand was
drawn over to cover the lime: the dampness in the sand acts as a reservoir of moisture for
slaking and helps to prevent "overburning" which damages the lime. Slaking
generates heat, and steam would rise gently from the pile. After a while, when the slaking
was judged to be complete, the pile was broken and mixed, and the result - if everything
had been judged correctly - was a fine and intimate mixture of dry hydrate of lime
and sand, the heat of slaking having helped it to dry.
The next step was to "screen" the dry mixture. This was done by using
a shovel to cast or "punch" it at an inclined wire screen with a mesh of perhaps
a quarter of an inch, thus screening out larger lumps, most of which would have been the
underburnt "cores" of lumps of lime, which would not slake. After screening, the
dry mixture would be knocked up with water, mixed with a "larry" - a sort of hoe
with holes in the blade - and then was ready for use as required.
When I read Gerard’s paper I immediately felt that it had the "ring of
truth", and it solved so many problems. Of course, it had all been there in the older
textbooks, but we just hadn’t seen or understood it. Richard Neve, in The City and
Country Purchaser (1703), says: "The best way ... is to take the sand and throw
it on the lime whilst it is in the stones before it is run, and so mix it together, and
then wet it. ... When you slack the lime you must take care to wet it everywhere a little
(but not overwet it) and cover with sand every laying or bed of lime as you slack it, so
that the spirit of the lime may be kept in and ... mix itself with the sand".
Excited and intrigued by Gerard’s paper (which also contains some important
arguments about other aspects of lime) I offered him the opportunity to run a day school
at the Museum to explain and demonstrate his ideas. He enthusiastically agreed, and
enlisted the help of his friend Douglas Johnson, of Mason’s Mortars, Edinburgh, a
leading lime expert and practitioner.
