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Frequently
Asked Questions & Glossary Q.
Why use lime on old buildings?
Before this century building techniques
and materials were very different from those employed today. Traditional properties
need to "breathe" to allow moisture inherent in a solid wall construction without
a damp proof course to evaporate from the external stonework or render. Many old
buildings are constructed from materials such as brick, cob and stone which are
relatively porous and often of lower strength. Lime mortars were normally used
for bedding and plastering. Lime mortar is a relatively softer mortar and therefore
it is able to withstand a certain amount of movement (without cracking) that comes
with settlement and seasonal changes in ground conditions. Lime mortar is porous
and allows moisture to evaporate, helping to keep a building free of damp. Q.
What is wrong with cement mortar?
Apart from the adverse effect it has
on the environment in general, cement mortar is usually hard, brittle and less
porous than lime mortar. It often contains additives to make it sometimes completely
waterproof and is damaging to traditional buildings for several reasons. Cement
mortar is often harder than old bricks, cob or some types of stone, therefore
when movement occurs it may damage these softer traditional construction materials.
Hard cement mortar can trap moisture behind it causing damage to the structure
and encourages ground water to rise up a solid wall by capillary action. Trapped
water in the wall can cause poor insulation, decay and crumbling. In severe cases
a cob wall can fail. The brick, cob or stone can also be subject to frost damage
if moisture levels are to high. Q.
Are these traditional lime products more expensive than modern cements and gypsum
plasters?
The materials cost a little more and some firms will also charge
a premium for lime work. We run a regular practical course and over the last 9
years many hundreds of owners have been inspired to tackle many repairs themselves.
In the long run using the right materials is less expensive than causing damage
to the building. Q.
I am new to traditional products, how can I find out about what lime products
to use and quantities needed?
We have comprehensive guidance notes on
our website which can be printed out. We run regular one-day practical courses
on the use of traditional limes and we also sell a training video or DVD. We are
also more than happy to give advice over the telephone. Q.
Can I get on site advice about problems or materials?
Yes, we carry out
site consultations on an hourly rate of £30 per hour + VAT, door to door. Q.
When should I use a natural hydraulic lime or non-hydraulic lime?
Natural
hydraulic limes set even when wet and come in a range of strengths. They are useful
for building with stone or brick where the earlier set may speed up construction.
You can also add a pozzolan such as metastar to a lime putty mortar. We suggest
that for external bedding and pointing late in the year natural hydraulic lime
should be used. It is capable of a faster initial set in cold weather. For most
external rendering and internal plastering jobs, the fattiness of lime putty makes
a superior mortar that allows coarser sands and thicker coats to be applied without
shrinkage. Q.
What is quick Lime?
This is the raw material that is used to make lime
putty. Quicklime is made by burning limestone or chalk in a kiln. This drives
carbon dioxide into the atmosphere, leaving a very reactive material called quicklime
(calcium oxide). It is made in a range of sizes from lumps down to very fine powder.
We use the finest grade of high calcium quicklime from Buxton, Derbyshire. Q.
What is lime putty?
This is quicklime that has been slaked. Lime putty
is sometimes also called non-hydraulic lime or 'fat lime'. Q.
What is slaking?
It is the process for making lime putty. Quick lime
is added to water and a chemical reaction occurs resulting in the release of a
large amount of heat from the quick lime creating a boiling liquid. The liquid
is drained off through a sieve into a settling tank. Chemically the calcium oxide
is converted into calcium hydroxide. The longer the slaked lime is matured in
the tank the better the lime putty. We leave ours for around 4 months. Q.
What sands are suitable?
For lime putty mortars, the sand used for building,
pointing and backing coats of render and plaster should be a washed sharp coarse
sand. We use a sharp sand to BS882 which is free of vegetable matter, clay and
salts. For finishing coats of internal plaster we mix a very fine sand with the
lime putty. Q.
What is lime mortar, or coarse stuff?
These are mixes of lime putty and
coarse sand, used for building, pointing, rendering and plastering. The mix for
mortar is normally 3 or 3.5 parts sand to one part lime putty by volume. Q.
How long can lime mortar be stored?
We supply our premixed lime mortars
in sealed dumpy bags or sealed tubs. It will last indefinitely as long as long
as air cannot get in to carbonate the lime. This means keeping it from drying
out and storing it free from frost. Mortars with hair teased in will gradually
lose the hair as it dissolves in an alkaline wet mortar and you will end up with
an unhaired mortar. Mortar left for many months will take more "knocking up" to
plasticise the lime mortar. Q.
Should I protect lime putty or mortar from frost?
Both lime putty and
lime mortar should be protected from frost. If frosting should occur, it is possible
to reuse frozen lime mortar so long as it is completely thawed out and thoroughly
mixed before use. Q.
What is haired lime mortar?
Animal hair, usually horse, cow or goat,
is added to the backing coats of lime mortar used for external rendering or internal
plastering onto stone, cob, brick or timber lath. Adding hair gives extra strength
and minimises shrinking and cracking. It allows thicker coats to be applied to
uneven walls and holds the plaster keys in place when plastering onto lath. Q.
What is lime plaster?
This is a finishing plaster used as a final skim
internally. We supply three grades of lime plaster - 2/1, 3/2 or 1/1 which are
the ratios of a fine sand to mature lime putty. The 2/1 grade is suitable for
plastering onto haired coarse float coats, the 3/2 is better for feathering into
existing plasterwork and the 1/1 is appropriate for matching the finest historic
finishes. Q.
What is Limewash?
Limewash is a traditional, breathable paint made from
lime putty which is thinned with water. Our limewash is approximately 1 part mature
lime putty to 1 part water. Other suppliers make much more watery limewash. Limewashes
are coloured with pigments and can be used internally or externally on lime plaster,
lime render, stone or brick. It works best on porous surfaces and hardens as it
absorbs carbon dioxide from the atmosphere to form calcite crystals, giving it
its unique appearance. Typically four coats are needed on new external render
and three coats on new lime plaster. Q.
What is a pozzolan?
This is a powdered additive made from burnt clay.
It reacts with the lime in a mortar to create harder chemicals and so is very
useful for damp or frost-prone environments. Q.
What is Natural Hydraulic Lime (NHL)?
This is made from limestone that
contains impurities such as clay or silicates. Unlike lime putty which is non-hydraulic
lime, NHLs can set in damp conditions. They also have some free lime available
for carbonation. There are three European classifications NHL 2, NHL3.5 and NHL5
based on the compressive strength of laboratory mortars after 28 days. These are
often somewhat misleadingly termed feebly hydraulic, moderately hydraulic and
eminently hydraulic. We generally recommend the use of NHLs where the need for
breathability and lower strength is outweighed by the desire for an earlier and
harder set such as working on wall copings, chimneys and slate floors. Q.
What is wattle & daub?
Wattle are strips of wood, often hazel, woven
between timber studs in a timber frame house. A mixture of clay and chopped straw
was used as the plaster to coat the wattle. Q.
What is lath?
These are thin strips of wood used as a key for the lime
plaster. Chestnut, oak and larch are popular choices of timber for the lath. Riven
lath is hand split along the grain using traditional tools and provides extra
key on the surface of the lath so it is best for repairing lathed ceilings. Sawn
lath is straighter with a less textured surface than riven is best for repairing
lathed walls. In traditional buildings the lath would be lime plastered and limewashed.
Lath can deteriorate over the years due to insect infestation or rot, especially
if modern less breathable plasters or paints have been applied. Q.
What are cob blocks and cob bricks?
These are made from a clay-based
sub-soil with straw. The cob blocks are about the same size as a concrete block,
the cob bricks are a little smaller than a modern brick. They are used for repairing
holes and damage to cob walls as they are already dried and hence will not shrink
in use. They are bedded in place with a 4/1 lime mortar or sieved sub-soil. Q.
What is render? A coating of lime (or cement) mortar, usually referring to
the outside of a structure. Q.
What is plaster?
A coating of lime (or cement or gypsum) mortar, usually
referring to the inside of a structure. Q.
What is pointing?
Filling the gaps between stones or bricks in walls
with a lime (or cement) mortar. Q.
What is harling? - also known as Scat Coat, Thrown Coat or Rough Cast
Harling is a technique for applying a coat of render or plaster. The lime mortar
has extra water added to bring it to a runny consistency. It is then cast on the
wall using a special tool called a harling trowel which has a curved blade. It
provides a rough texture when cured that gives extra key for the next trowelled
coat and also controls suction with very thirsty materials such as cob and brick. BACK
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