Quicklime: Hot Mix

This blogpost is part of my ‘limestone lockdown’ project. For an Introduction to the project, and a guide to the growing list of related posts, see Limestone in the Lake District: an Introduction – and the ‘categories’ list in the right-hand bar.

It’s May 2021, the latest lockdown for Covid has been eased and crossing the Border between Scotland and England is once more permissible, so I drive North to Canonbie where Alex Gibbons has his yard. I’ve known Alex since 2016, when he was designing and overseeing the construction of the demonstration clay dabbin house at RSPB Campfield near Bowness-on-Solway, as part of the Solway Wetlands Landscape project. On one occasion we spent a memorable morning doing ‘experimental archaeology’, as Alex laughingly referred to it; he had read that the use of fresh ox blood was a traditional way of making a glossy, hard-wearing floor, but abbatoir rules meant that obtaining a bucket of blood was not an option. Instead, we mixed dried ox blood with earth and water attempted various ways of spreading and painting it. (A warning: the method was not effective – subsequently the floor became smelly and furry with mould as it absorbed the damp.)

Alex and volunteer helper John Lackie at the Campfield dabbin house 2017

My instructions are to turn up a lane between a farmhouse and a ‘tin shed’. The lane turns out to be a rough farm track with a slalom course of deep potholes; a man and a border collie, busy attending to a quad bike in the yard, both appear surprised when I drive by. Alex, a Fellow of the Society for the Preservation of Ancient Buildings (SPAB) and an expert in constructing and repairing earth and lime buildings, runs his own business, ‘Stick in the Mud Conservation’, and he’s invited me to his yard to watch a ‘hot mix’, the process of making a large batch of lime plaster from quicklime. The yard is based next to a longhouse that he has almost finished renovating. Ambling out in response to my shout, smiling, with unruly curly hair and plaster-spattered work clothes, he explains, with a grin, that this is the best example, with its cruck frame and cobbled floor, of a dabbin house in Scotland – because it’s the only one remaining! (For much more about the dabbin houses of Cumbria and Scotland, see ‘What’s a Clay Dabbin?‘)

On one side of the yard are large white dumpy bags full of quicklime, which looks like dirty-white grit. Alex buys this ‘kibbled’, that is crushed, lime from Tim Wells of Eden Hot Lime Mortar, who obtains it from TataSteel’s vast kilns at Shapfell Quarry next to the M6. Some people buy the quicklime already-mixed with a small amount of water, but Alex prefers to mix it himself: “I have the raw materials – the quicklime, the sand. I just like doing it, really, and I can adapt the mix myself depending on what I’m using it for – even though it costs me in terms of labour.” The grit sand, piled up by the hedge, comes from the Cardew Mires Quarry at Dalston near Carlisle. It’s grey-brown, and has a coarse texture with tiny sharp bits of grit; it needs to be “chunky, to hold its own shape.”. The quarry sometimes also supplies Alex with material for his clay dabbin work.. “I have to check it to make sure it’s right, which I know sounds ridiculous, but it can go wrong! I can get 20 tonnes at a go, which is ace!”

We walk across into one of the open-fronted buildings, squeezing past a blue pickup and low-sided trailer into a large corrugated iron shed. Piles of straw, white plastic containers of water and of sand, bulging plastic sacks, folded tarpaulins and plastic sheets – the plethora of basic materials for earth and lime building – fill the space. There’s a hose-pipe snaking across the floor, and a wire leading to an old red van outside. At the back, in its own territory, is the plaster-encrusted mixer.

Now it’s time to make the hot mix. I’m wearing old clothes, waterproof trousers and now, too, safety goggles. “When the water’s added it fizzes and pops and steams – it’s sensible to keep back,” Alex warns.

The mixer is a roller pan mixer – there are two large rollers or wheels that crush the quicklime, and a paddle which turns and folds the mix. Alex fires up the generator in the back of the red van and the machine starts rumbling as the axis turns horizontally, the wheels turning on vertical axes (‘the wheels help no end’). For the lime plaster he’s making today, the mix requires two buckets of water, one bucket of quicklime and four buckets of sand; the proportions will vary depending on the use. When he adds the water to the dry ingredients, clouds of steam gush and swirl around us. Lumps of the mixture are adhering to the edges of the rollers, and Alex scrapes them off with a spatula which was hooked onto the wall next to the machine. Next he picks up a bundle of goat hair, and pulls out the fibres, sprinkling them into the mix. “Traditionally it was usually horsehair, but that’s not easy to get. The goat-hair is soft, it has barbs” (which help the adhesion between the fibres and the lime). It is imported from China, having previously been treated to kill anthrax, a treatment which has also removed some of the natural grease.

We stand watching the mixer. It’s mesmerising, I say, I can’t look away.  “I always warn everyone,” Alex laughs. “Don’t look into the mixer, you’ll get sucked in!” It is a surreal scene: in a dimly-lit shed and half-hidden by steam, the machine grinds and groans, so that we must shout to be heard, and outside, from a darkening sky where heavy black clouds are louring, rain begins to fall. There is a strange, sharp smell coming from the slaking lime.

Alex opens a shutter at the bottom of the drum and the pale-brown mixture slides out into the waiting, plaster-caked wheelbarrow. He scrapes down the shute, closes the shutter, leaving the machine mixing – then runs the barrow of plaster up into the waiting low-sided trailer and tips it out. The rain is pock-marking the sloppy mess of plaster, so he unhitches the trailer and pushes it further into the shed. The plaster in the trailer is steaming; the slaked lime is hot and very caustic – he shows me scabbed burn marks on his arms, where small gobbets of mixture had landed previously, and grimaces, “It can be very painful.”

For the second of the four batches he adds less water and the reaction is more dramatic releasing greater clouds of steam. “A lot of it is about feel – you add water until it looks and feels right. If things go wrong you can always sort it out!” Does he learn by experience? He laughs: “You learn by cocking it up, like that first lot!” (It was too sloppy – the subsequent batches are much firmer).

And he surprises me when he says, “There’s no part of this job which isn’t really hard work. It’s very physical. It’s like thatching – that always sounds romantic until you end up doing it!”

While mixing the fourth batch, the machine abruptly stops, jammed. Alex yanks at the paddle, then stirs and pokes the mix with a long-handled shovel (which had beens propped nearby, exactly for this purpose). He switches the mixer on and off several times; the generator whines and grumbles. The mixer groans and stutters, recalcitrant, but finally starts moving, gaining speed.

We chat about his work and lockdown. He tells me he is now doing much more teaching, particularly for the Prince’s Foundation on a new dabbin at Dumfries House. He also got married about ten days before our visit; the last time I met him he was living South of the Border near Wigton, but now he lives in Scotland, just North of Canonbie. This trailer-load of lime plaster is to be used to repair the wall of a nearby walled garden.

This is a characteristic of lime plaster, that it can be used indoors or out. And I’m surprised that he will not be working with it until the following day: would it not dry out? He explains, “You can age it, and the older the better. The bits that are not completely slaked get slaked overnight. It’s more workable the next day too.” Even more surprising is that if you cover the plaster in a bucket with a layer of water, it will remain usable for a long time – it doesn’t dry and harden, taking up atmospheric carbon dioxide (see diagram of the Lime Cycle below) until it is spread.

Testing render on the dabbin house back in 2017

The rain has eased, the trailer is half-full of porridge-y plaster, and Alex is ready to head home. Before I leave he tells me to check on the clay dabbin house at Campfield, where he has just finished replacing the lime-render on the gable end which had suffered over the winter.

“And you should go and see Tim,” he says, “A nice man. He likes talking about lime.”


The Lime Cycle

Tim Wells lives in the Westmorland Dales, in a handsomely-renovated farmhouse near Great Musgrave. I arrive in the village and, after a couple of phone calls as we each try to ascertain where the other is, I find his low-loader JCB waiting at the end of a concrete drive, and follow him to his yard. His family ran stock on the farm – 700 sheep and 300 cows – until the 2001 Foot-and-Mouth epidemic that saw their animals culled out; like so many farming families, they have had to diversify and seek new occupations. Tim decided to learn about the renovation of old buildings, and eventually set up Eden Hot Lime Mortar to supply materials and expertise. As a business it’s thriving and increasing each year; he tells me later that he receives 50-60 calls a day, fixing meetings and starting conversations – “getting builders on board, and architects, and importantly insurers. It will become massive with the next generation”, as people get a better understanding of how to renovate older buildings.

The yard is large, with several barns and sheds, storage areas, and concrete clamps for sand and grit. Tim comes over to greet me and I get an impression of strength; he has a shock of dark hair, a broad, tanned face, and strong, tanned arms. At first he speaks in short sentences, slightly terse explanations of what we’re seeing, but he becomes more expansive as we both relax. A small black dog, Connie, follows us, roots around in some black polythene, then lies down in the corner of a sandy clamp, and generally keeps an eye on us.

Hopper and mixer

The quicklime Tim obtains from the Shapfell kilns has already been through a crusher, and comes in various sizes; he prefers the 2mm pellets. He sells this as it is (these are the pellets Alex uses) or – he shows me a row of 1-tonne dumpy bags – he mixes it with grit and some water to make partially-slaked lime. The mixture is crumbly, but no longer caustic by this stage, and the buyer then adds a small amount of water to make lime plaster. “It will keep for ten or more years like this,” he tells me. “You can re-mix it and use it.”

Nearby is a very large mixer with rollers and paddle, which can be filled from a hopper; this is a large-scale operation, for not only does he supply materials for builders, but he also runs between 50 and 60 training courses a year. “We get trainees from all over, Orkney, Shetland, London, Hampshire … They come for a day. I hand out pdf sheets of instructions and they get to try different things.” In a shed by the mixer are rectangular wooden frames filled neatly with limestone blocks, ready for mortaring and pointing; slates laid on top of laths form a mock roof for pargeing; there’s a vertical array of laths for lath-and-plaster training. On a table are tubs and packets of pigments with names like Burnt Turkey Umber, Brown Umber and Venetian Red, for colouring the plaster, and cylindrical rolls of the goat hair – he pulls some out of the bundle, teases it apart and snips it with sheep shears to show me the size for sprinkling into a plaster mixture. The hair gives tensile strength and forms a composite material, which is useful for crack-stopping. “But people used to mix the lime with whatever was to hand locally, crushed oyster shells, wood ash, ash from the steel works – nothing was written down, the knowledge was just passed on.”

Modern mortar uses cement, based on a mix of calcium carbonate and other minerals such as silicates; modern plaster is based on gypsum, hydrated calcium sulphate; both materials have the advantages of standardised components, and are quick-setting. But anyone who renovates and repairs old buildings is quick to explain how these modern materials are completely inappropriate in that they do not allow the walls and floors to ‘breathe’ or flex. Water is trapped and walls become damp; cement and mortar are rigid and can transfer cracks to the surrounding walls; modern plaster may seal in the damp. As Tim says, “These modern houses won’t last fifty years!”

The well-designed and informative website for Eden Hot Lime Mortars gives several reasons why hot lime mixes should be used instead: “Walls breathe better and moisture can evaporate. Mortars and renders do not set too hard. Thermal movement can be accommodated without damage…”

There is much more of interest to learn about lime mortars, such as the difference between hydraulic lime, air lime and pozzolans, but I’m beginning to suffer from information overload. I’m saved from embarrassment by Tim’s phone ringing: he has another appointment lined up and his visitor is trying to find the yard. Connie, the little black dog, accompanies me to my car and jumps onto the driving seat when I open the door; I scoop her out, regretfully, and thank her for the dusty pawprints she has left as a reminder.

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