[Contents]
Gypsum plaster has its place in the rush-rush, hurly-burly, fast-moving world of mass-produced, slap-it-up-quick and move-on-to-the-next-job-tomorrow, building industry. So let's leave it in that place.
Lime allows you to be slow. To take time. Have another cup of tea. It all right, relax, it won't go off. If you don't use it all up today just cover it with water and it will be fine in the morning.
Lime can go on the outside, call it render, or indoors, then it's plaster, but it's all the same stuff. (You can't hang gypsum out in the rain.)
Recipes. Before reading further take a digression to this useful article by Ian Constantinides. He explains how a great variety of mixes has been traditionally used, and it probably doesn't matter much.
I've been experimenting with various mixes recently and have come to the
conclusion that for much general plaster work (I'm not talking fine cornices)
the following mix has certain advantages:
Six parts coarse sharp sand passed through a 10mm garden sieve
One part clay-rich subsoil
One part dry bagged hydrated lime from builders' merchant
(Use a finer sieve for the top coat.)
I mix a wheelbarrow full at a time, dry, and then add as little water as
possible to make a workable mix. Then add a bucketful of chopped straw and mix a
bit more.
Advantages:
Cheap; being half the price of a 3:1 mix (I dig up the aggregate for free)
Eco-friendly; half the embodied energy and CO2 production of a 3:1 mix (no
energy/CO2 cost of aggregate dug up withing wheelbarrow range)
Traditional; vernacular building relied on the cheapest local materials
available. Over-specification is the bane of modern building.
It works. Really.
There is a good theoretical reason why a sand/earth/lime mix is better than just
lime and sand. The aggregate should be of a range of grain size, the coarsest
grains just touch each other, sand fills the spaces, silt fills the spaces
between the sand grains and clay fills the spaces between the sand. The less
rounded and more angular the particles the better because they lock together
instead of slipping past each other. Clay minerals are extremely angular and do
such a good job of locking silt particles together that most of the world's
houses are made of earth and actually stay up better in earthquakes than
concrete houses. Adding a very small proportion (one eighth of the total by
volume in the recipe above) allows calcite crystals to grow amongst the clay
minerals, firmly fixing them together. The water adsorbed by the clays allows
the growth of calcite crystals to proceed further, as the hydrated lime
carbonates, than if there were no clay present.
I use a wooden float because I can make it myself. Some folk recommend a plastic
one but that requires money and a petrochemicals industry. Steel floats, the
normal tool of the gypsum plasterer, gives a very smooth finish and if you work
it too much drags lime to the surface, removing it from the layer just below,
which consequently is weakened. If you do want a glassy smooth finish to your
lime plaster, squeeze it with a glass object such as a jam-jar a few hours after
plastering. You can achieve a very high polish if you work at it.
Contact: biff@biffvernon.freeserve.co.uk
Lincolnshire
©Biff Vernon 2006