Hi Frank,
Some years ago when I/we refurbished a venerable Northumbrian
Lectern Dial I had occasion to do a little minor research on lime putty and
transcribed the following notes from textbook on stone although I don't recall
the title.' The lime seems to have survived the burning process in chunks so
would be easily sorted from the ash perhaps? I recall my father telling me
about building sites in the early 1900's where the post slaking was done in a
wood-lined pit in the ground.
Best Wishes
Tony (Moss)
"From early times until well into the nineteenth century the preparation of
the lime was no easy matter. It can be obtained from any of the limestones,
including chalk, which today provides about a third of the total quantity of
lime used in the building trade; but the general belief is that ‘stone lime’ is
superior to ‘chalk lime’. Some of the best is made from Blue Lias limestone.
The first step was to convert the lime into quicklime, and to do this it was
necessary to burn it. The old kilns were little more than large circular holes
in the ground, fired by timber and brushwood covered, wherever it was
available, with a layer of coke. Over this was placed a layer of broken-up
limestone, and then more coke and so on in alternation; the kilns were
about ten feet deep. The fire was then lit, and as the layers of coke were
consumed the burnt lime dropped to the bottom. The situation is very
different today; the process is now carried out under scientific control in
enormous steel kilns lined with firebricks, coke, oil or gas fired, and
continuously burning.
The burnt lime removed from the kiln, quicklime, or lump-lime as it
was called in some areas, was now in a volatile state. This incidentally
rendered transportation very difficult. Before the Industrial Revolution
heavy materials could only be carried over long distances by water. But
quicklime could not be put into barges, because with the slightest
dampness it would start to heat and slake, and that might mean serious
damage, even the burning and sinking of the vessel. So the limestone had
always to be shipped unburnt, and the kilns erected close to the building site
or, if that proved difficult, at least on the quayside at the point of
disembarkation. When quicklime was loaded on to carts or wagons, and
later on to railway trucks, these risks still had to be countered. Only with
the development of hydrated lime was this difficulty overcome.
There were two main processes for slaking lime: the liquid and the semi-
dry methods. In the former, lumps of burnt lime were carefully placed in a
large, generally circular, tub or tank half-filled with water and stirred. ln a
short time, as it absorbed the water, the lime became hotter, and with more
stirring it would boil from its own nature, without the application of any
other heat. ln this state it could be dangerous; above all the eyes
needed protection from this bubbling white liquid. More lime would now
be added until, with yet more stirring, the liquid became thicker and
acquired a creamy consistency. It was then ladled out and poured through a
fine sieve into what was called a pan. This would be either a shallow hole in
the ground or a container on the ground built up with sand to a height of
perhaps eighteen inches and lined with sand at the bottom. In this the liquid
had to be left to cool for a week or two. During this period the water would
gradually evaporate. When it had reached about the consistency of butter,
it was known as ‘putty-lime’, and was now ready for making into mortar.
Some people, however, were in no hurry to do this, believing that the
longer it was kept in stock the better would be its quality since any
fragments of lime that had passed through the sieve inadvertently would
have time to ‘blow’. If this only occurred when the work was finished it
could be serious; in bad cases, indeed, ‘blowing’ might cause a mortar bed
to ‘lift’ or swell even to the point of dislodging a block of stone. So the
putty-lime might be left to stand for a year or even more. All that was
needed at this stage was to mix it with sand.
In some parts of the country the semi-dry process was preferred. The
first move here was to spread out a layer of sand, on to which were placed
alternate layers of quicklime and sand; each layer of lime was then watered,
but sparingly, because what was required at the end was a dry powder.
Gradually a conical heap was formed, which might be five feet high and six
or seven across at the base. After the final watering, the whole was covered
with a layer of sand and left to heat up of its own accord, expand and blow.
After being thrown through a fine screen, it was ready, by the addition of
more water and sand, to be converted into mortar as required.
Most limestones are not naturally hydraulic: that is to say, they will not
harden when wet. Indeed, the purer the limestone the less hydraulic is the
mortar derived from it. This explains the strange additions of early times,
and accounts for the frequent introduction nowadays of a little Portland
cement to temper the mortar and harden it.
But there are varieties of lime in Sussex, Cambridgeshire, Leicestershire
and elsewhere which are hydraulic, and have a reasonable tensile strength;
and these are the best for the production of mortar, not only because they
set more firmly and have the colour of natural stone but also because they
are free from those sulphates and other substances which can cause surface
staining or other defects.
The Collyweston stone slaters (see Chapter 8), who bed their slates on
the roof, used to make their mortar with limestone from their own pits.
This was mildly hydraulic, so when they added the sand, very little water
was needed. To obtain a workable consistency they used to beat the
mixture with flails, not unlike those formerly used for threshing corn.
After the First World War the situation was very much eased for the
stone builder by the introduction of manufactured hydrated lime. Mixed
with specified quantities of sand or stone-dust, this lime can be used
straight."
-----Original Message-----
From: Frank Evans <[email protected]>
To: Sundial <[email protected]>
Sent: Sat, 4 May 2013 15:43
Subject: quicklime
Greetings, felllow dialists,
At the recent meeting of the British Sundial Society in
Edinburgh I spoke on the conservation of the 1793 stone dial on
my local parish church. This conservation work was being carried
out at the time and I learnt from the stonemason involved the
importance of lime mortar in stone preservation as opposed to
cement.
Most people have seen early lime kilns and know that they produced
quicklime to be subsequently slaked, to be used as land fertiliser or
lime mortar. We know that the kilns were charged with limestone
and with a fuel of wood, coke or coal. My puzzle was, what
happened after the couple of days of ignition. Inside the kiln would
be quicklime and ash. How was it separated? And once separated, what
happened to the quicklime? While the evidence of the kiln function is
obvious, how and where did the slaking take place (we are talking of
several tons in each batch)? It had to be slaked on the spot,
being dangerous and untransportable stuff. The slaking process
takes several days to complete. Was there an area set aside for iron
pots to contain the boiling reaction? I have seen no evidence of any
special area. All of the authorities I have consulted give comprehensive
accounts of the roasting but none of the equally important
slaking.
Lime mortar is a material of importance to dialists. Although cement
was first produced in the nineteenth century it was not until as late
as the nineteen seventies that the superiority of lime mortar
over cement for stonework was fully realised. For repairs and
pointing to stone dials lime mortar, which is flexible, should always be
preferred to cement. (Cement is to be preferred for brickwork).
The two mortars differ in that cement mortar sets with water while
(non-hydraulic) lime mortar sets in air, taking several months to be
fully finished.
Can anyone throw further light on this, please.
Frank 55N 1W
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