HPH230 : What is lime render? – with Martin Brown from Best of Lime
Martin Brown explains how this ancient material can be used in the modern world.
Interview with Martin Brown
A decorator by trade, Martin spent time working in the family business specialising in handprinted papers and on traditional buildings. That led him towards lime plaster with a particular interest in some of the old, traditional mixes that have been lost. He has been running his Best of Lime company since 2014.
Lime is a binding material
Lime is burnt calcium carbonate. Chalk, limestone, shells can all be fired in a kiln to make quicklime which you then slake (combine with water) and use as a binder.
France has a long tradition of manufacture, with the Bordeaux region being a big producer, and in the UK large quantities are made mostly in the Buxton area, but also in Lincolnshire which produces more of a chalk lime.
It uses a lot less energy to make than cement
Martin believes that lime has a number of advantages over more modern materials.
For a start, cement is fired at 1300 degrees, whereas lime is between 900 and 1000 degrees, making it a more sustainable product.
Aesthetically it is arguably more visually attractive, it is more flexible and is more breathable allowing free transport of moisture through the building, creating a healthier environment.
Different additives provide reinforcement
Lime has been used as a binder for several thousand years, albeit with different tweaks and additives to make it perform differently. And while recipes from the 1830s that included bone ash, urine, beer and cheese in the mix have thankfully been superseded, animal hair, which acts as a reinforcement, continues to be popular! Horse hair is popular but cow hair is evenly more commonly used, particularly in the chalk plasters of East Anglia. A sand and lime mix on a green oak timber frame will crack and fail as the frame twists and shrinks, but a lime and chalk mix with lots of cow hair gives a mix that has plenty of flexibility. It can also be applied very thickly, and stands up to the rigour of both hot and cold temperatures.
Some plasterers see it as a dark art!
Martin's aim at Best of Lime is to bring an ancient material into the modern world, and make it perform as well as, if not better than, modern materials. For this to happen it also needs to be easy to mix and apply.
All their plasters can be mixed using an ordinary plasterer's paddle drill or standard bell mixer.
The dry mix is combined with water and a separate bag of fibre reinforcement. The fibre they use is 3 denier, which equates to around 280 miles of fibre per 25kg bag.
Having that much fibre is something that couldn't be achieved with traditional methods such as animal hair or screeding fibres which are much thicker. It is mixed into the top coat where, thanks to it being so fine, it isn't seen.
Lime lends itself to rounded corners because it's a softer material, and the slower setting means there's time to mould it into the desired shape.
“It has an organic soft aesthetic to it. So, when it is finished, it has a much softer look to it. And of course, you’ve got the mechanical side of it being a much more lightweight, less dense material than sand and cement. You don’t need to have movement joints everywhere. So, you have a much more traditional look to a building without big mastic joints all the way down it.”
Keep an eye on the weather forecast!
With proper planning, even if there is inclement weather the plasterer will hopefully have jobs still to finish off inside the building. While the autumn is a busy period for getting work done before the weather turns too cold and wet, modern work programmes don't tend to allow for downing tools until the Spring. It might be that a ‘tin hat' approach to covering the building is required to keep the rain off.
“So, getting the plaster to dry outwards, especially if you’ve got a building where it can’t dry inwards, if you’ve got a particular background that doesn’t suck the moisture back in, then we have to look at air movement, warm air if possible, covering in scaffolds. There’s lots and lots you can do but in the end, it’s all water based, and keeping water off.”
Martin says that his products stand up to being applied in extreme high temperatures, but that it's usually the plasterers who will give up in the heat!
There are a variety of colour options
The colour of lime is mostly dependent on the aggregate that it is being used with, and this can vary regionally. A mix using a very white sand will make a very light finish, whereas towards Norfolk there is a tendency for a more brown effect. Martin uses a lot of chalk which makes a neutral, creamy off-white plaster.
For different colours, aside from decorating it afterwards, tints can be used although this isn't something Martin recommends as there's greater potential for problems. Any slight change in aggregate colour for example might show on the finished product.
For a traditional finish, a limewash can be used, either with or without added pigment. There are also modern silicate paints which look and perform more like a masonry paint, but have the important characteristic of being breathable.
In terms of maintaining the finish, the durability will be dependent on exposure levels and detailing around the building. The limewash will gradually wear away so may need to have another couple of coats within four or five years. In more sheltered areas it may last much longer.
There are also clear, breathable coatings that can be applied if there is a requirement to maintain the natural colour of the plaster, which help with water shedding and keeping the dirt and organic growth at bay.
They are unable to compete on price with sand and cement
However, one of the advantages they have is that work can generally be done in two coats. For sand and cement, if the traditional look of rounded corners is required they tend to use three coats, so what you lose in money on the material, you gain with the labour saving.
Find out more
Visit the website of Best of Lime
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Download a transcript of the interview with Martin Brown.