[Contents]
Lime
This is another Great Conspiracy. Houses used to be built with lime mortar. This has been going on for a couple of thousand years at least. It works. Then, in the 20th century, builders stopped using it. They forgot how to use it, Or died. Young builders never learnt.
Let's start with some geology. Millions of years ago the shells of countless trillions of sea creatures accumulated into layers of limestone. This happened at various stages in Britain's to produce the Carboniferous Limestone of the Pennines, the Jurassic Limestones of the Cotswolds and the Cretaceous Chalk of southern and eastern England. There are minor differences in texture and chemistry from one rock formation to another but, essentially, its all the mineral calcite, calcium carbonate, CaCO3.
The limestone is dug up, crushed and burnt. Not so very long ago, this was done in thousands of little 'limekilns' all over the country. Look on a 19th century edition of an OS map and you won't go far without finding the words 'Limepit' or 'Limekiln'. Burning the lime drives carbon dioxide off turning the limestone into quicklime. To minimise transport costs before the packhorse and cart gave way to the lorry, lime burning was done in the quarries and as close to where it was needed as geology allowed. The quicklime was taken to building sites where the builders slaked it. Slaking means adding water, converting the calcium oxide t calcium hydroxide. The chemistry generates quite a bit of heat so it was found safer to fill a pit in the ground with water and then throw the quicklime in a bit at a time and stand well back. The resulting sticky white mass is called lime putty. It can be used straight away but for reasons which even today remain pretty obscure, the qualities of the putty improve with age. The lime putty is mixed with sand to produce mortar, sometimes known as 'coarse stuff' and used for brick laying and plastering. And this is where the chemistry gets really clever. Exposed to the atmosphere, the hydrated lime absorbs carbon dioxide from the air and turns into calcium carbonate, the same stuff as we started out with when the limestone was quarried. The process is slow, taking weeks or months, but gradually the mortar sets hard.
There's beauty in the cycle of limestone to quicklime to hydrated lime and back to limestone. Unlike Ordinary Portland Cement, (OPC) it's carbon dioxide neutral. The CO2 driven off in the burning is reabsorbed as the mortar sets. There's the issue of the fuel used in the kiln, of course. It's less than for cement making as the temperature doesn't have to be so high but what we need is coppice-willow fired limekilns.
Lime mortar is vapour permeable, breathable, allowing buildings to stay warm, dry and cosy without resorting to plastic damp-courses. It acts as a wick, drawing moisture out of the bricks and away to the air. OPC is less permeable, trapping water within the masonry. Lime mortar is not too hard and brittle. When a building moves lime mortar will accommodate the changes, Very small cracks develop and heal themselves by micro-solution and re-crystallization of the calcite crystals. OPC breaks with big cracks that let the rain in, so you need massive concrete foundations to try and keep the building still. Such foundations are not needed when building with lime, the house is free to settle and move.
Lime is great on walls. Indoors, its more permeable than gypsum plaster so lime-plastered walls are more breathable. The building stays dry without a damp-course. Of course it's no good sealing the plaster with plastic paint. But lime is versatile stuff. Limewash - that's lime putty with lots of water - is the basis of much of the paint to be used on walls. But that's another story. Lime plaster, mixed with a little goat hair, is the traditional stuff of lath and plaster ceilings and partition walls as well as a covering for masonry walls.
Ok, but I haven't got a quarry, or a limekiln, or even a slaking pit. What should I do if I want to free myself of my addiction to OPC? Lime can be bought in various forms. For most purposes the best the best stuff is 'lime putty'. It is sold in a 25 or 30 litre plastic tub. It is a white squidgy mass with a couple of inches of water over the top to keep it wet and stop the carbon dioxide in the air getting at it. In this state it will keep for ever and the connoisseurs say that, like fine wines, it improves as it matures. It is not usually sold until it is a few months old and cost more if you want one or two year old putty. It's not cheap at £15 to £20 a tub but you may be surprised by how much re-pointing you can do from a tub full. If you buy a pallet-full of 24 tubs you might get it for less than a tenner a tub.
To mix your mortar, the coarse stuff, you need about one part lime putty to three parts sand. Now we could get really precise about this. The theory is that in a pile of sand, the grains touch each other at just a few points leaving gaps between. Ideally you want just enough lime to fill up all the gaps but not so much that the sand grains are pushed apart and are no longer touching. Sand grains are made of the mineral quartz. It is much harder and stronger than the calcite of the lime. For maximum strength loads should be passed from one sand grain to the next, the purpose of the lime being to hold the sand firmly in place.
If you really feel the need you could work out how much lime to use by calculating the volume of gaps in the pile of sand. Put a measured volume of dry sand into a a bucket and add water from a measuring jug until it is completely waterlogged but there is no extra water on top. The amount of water you just added is the volume of spaces in the sand, the amount of lime putty you need. Unfortunately your sand is now wet so you'll have to let it dry out or get some more. The volume of spaces varies a bit depending on the size of the sand grains and particularly if you have a range of grain sizes. Small sand grains will fit into the gaps between large grains so you won't need so much putty. If, like most folk who aren't too sad, you're not too bothered about all this then just go for a 1:3 ratio. That's one part lime for three parts sand. It'll do. Not satisfied? Then read the aggregates section on the Lime Centre website.
While we're on the subject of sand, some will talk sagely of sharp sand and of soft sand, of sand that is washed and sand that is graded. I just dig it up from the garden, gradually enlarging the water features. Some is fine, some is coarse, there are bands of pebbles and occasional oyster shells. If I'm wanting a fine mortar, perhaps for plastering, I throw the sand through a garden sieve. I prefer not to wash the sand as the iron staining gives the finished lime mortar a warm colour which is lost if the sand is given a rinse.
Mixing lime putty with sand is a bit of a labour of love. Well, more labour than love, actually. First pour off the water from your tub of lime putty, but into a clean bucket. Don't throw it away, you will need it later. Then scoop out the required quantity, maybe with a smallish builders' trowel, into the container you are using to measure your one part lime and three parts sand. Tip the lime putty and sand onto a board and start mixing with a spade. The sand should not be too wet and you should not need to add any water unless the sand is really dry. Chop the heap about and squeeze it back and forth with the back of your spade. It takes ages and you feel you are getting nowhere and then you notice that instead of loose sand with stiff white lumps in it you have an even, grey, smooth mortar that feels lovely. Now shovel the lot into a big bucket or tub, squidge it down a bit and pour a little of the lime water on top. The rest of the lime water goes back on top of the lime putty that you haven't used yet. Remember, the water protects the lime from carbon dioxide in the air. So long as it's under water it will last forever. It's best to leave the mortar overnight or longer, then, when you come to use it, pour off the water, give it a stir and away you go. I'm usually not that well organized, or I run out of coarse stuff part way though a job and have to mix up some more and use it immediately. It's fine.
One big problem with lime putty is that not many places sell it. Real lime putty is often only stocked by specialist firms dealing with conservation materials. My local stockist is 60 miles away. So what's the deal with bag lime, the stuff that every builders' merchant keeps? This is what is called 'hydrated lime'. Its made by adding just enough water to quicklime to turn the calcium oxide into calcium hydroxide but with no spare water to make it wet. Chemically it is the same as lime putty but it is a dry powder instead of a wet paste. It's not so heavy to transport because there isn't all that water and it can be easier to handle. But, and it's a big but, if the air reaches the lime it will start to carbonate, ruining the main reason for using it. So, buy it from a builders' merchant that has a quickish turn round, keep it sealed in the bag and then use it as soon as possible. It can be mixed dry with sand and then water added to make mortar. Alternatively, and preferably, tip the bag full into a plastic dustbin half full of water. Give it a good stir, let it all settle and, lo and behold, you have lime putty, at half the price of the ready-made tubs.
Now the purists will say it's not so good. And they're probably right. There may have been some in bag carbonation and the powder may not be as finely ground as desirable. If you can, leave your bin full of hydrated lime and water for ages to 'mature'. Lime putty maturation is about as mysterious as with wine. And terroir is similarly significant. What quarry the original limestone came from determines the few per cent of the material that is not actually calcite. Some lime putty producers proudly pronounce that theirs is the product of the purest Chalk whilst others declare for Derbyshire Carboniferous. Ah well, remember that for many centuries builders used whatever was local. The local vin-de-pays tastes best when the sun's out.
And when you've built up a good surplus of empty bottles they can be crushed and used in the lime instead of sand. Details. More drinking, less quarrying.
Suppliers of real lime putty:
Bleaklow Industries Ltd, Hassop Avenue, Hassop, Bakewell, Derbyshire. DE45 1NS Tel: 01246 582284 Fax: 01246 583192
Hirst Conservation, Laughton Sleaford, Lincolnshire NG34 0HE 01529 497517
Mike Wye & Associates, Buckland Filleigh Sawmills, Buckland Filleigh, Beaworthy, Devon EX21 5RN, UK 01409 281644
Calch Ty-Mawr Lime The Welsh Centre for Traditional and Ecological Buildings. Llangasty near Brecon. 01874 658249
The Lime Centre Long Barn, Morestead, Winchester, Hampshire, United Kingdom SO21 1LZ, 01962 713636
Ryedale Conservation Supplies The Howardian Hills, North Yorkshire
to be continued...
Contact: biff@biffvernon.freeserve.co.uk
Lincolnshire
©Biff Vernon 2004