Agriculture Reference
In-Depth Information
characteristics of a particular lime in its preparation. Some slaked grey limes,
even after running through the final fine-meshed sieve into the dipping box,
could be a little gritty and this performed as the aggregate. As lime mortars
expert Bob Bennett states:
Grey Lime was a relatively impure material as it contained tri-calcium sulphates
and silica. Further random contaminate could also be found in the form of wood
or coal ash from the old brick or stone kilns and, occasionally, small traces of
alumina have been identified. I believe that these impurities contributed to the
performance of grey lime in two ways. Firstly, some of the contaminates act as
pozzolans when into pure, non-hydraulic lime mortars. Secondly, Grey lime mor-
tars were mostly prepared in a 'pan-mixer' with rollers, which crushed any large
particulates to a predetermined fine size, coincidentally acting as an aggregate.
Lime is usually the 'binder' in a mortar, the sand being the 'filler', so as the
joint size of gauged work thickens the ratio of sand has to increase. On an ash-
lared joint of 5-6 mm (¼ in) the ratio is likely to be 1:2 or 1:3. Without the sand
there would be a loss of mechanical strength, an increased likelihood of crack-
ing and crazing across the joint width during drying-out; and long-term failure.
Apart from these structural considerations sand compared to the binder was,
and remains, very cheap and thus keeps material costs of any mortar down.
This problem of craft terminology, relating to lime putty, leading to confu-
sion with modern readers, was raised with master plasterers Alex Hyland from
Scotland and Jeff Orton from England. Both agree this misunderstanding occurs
in plastering too in respect of the final 'setting coat' of fine stuff that is often
wrongly being thought of as consisting of lime putty alone. As Jeff Orton states:
The overall thickness of the final setting coat, like the joints on gauged work,
can be from approximately
1
⁄
32
to
3
⁄
16
(1 mm to 5 mm) according to the type of
finish…
The proportions of lime to sand can vary between 3 parts lime to 1 part of sand,
to 1 part of lime to 3 parts of sand, or any combination in between. The mixes
containing more lime than sand are the softest, and are used on ceilings etc. The
mixes containing more sand than lime are the hardest, being used for walls and
areas likely to suffer from knocks and abrasions. The more sand the mix has in
it, the better the process of carbonation has in taking place, along with the pres-
ence of moisture in the atmosphere, consequently the more durable it will be. In
some areas of this country it is not uncommon to find crushed limestone, sieved
and used as an aggregate instead of fine sand, but this does not always show-up in
analyses, just that it all is calcium carbonate.
Miller (1897, 97) states that in America they call this setting coat the 'Putty Coat',
and later in his book he had this warning about using lime putty on its own
(Millar, 1897, 100):
