Monday, 29 August 2016

Re-pointing the chimney breast with lime mortar

I'd like to start this post by saying that there aren't many jobs in The Lodge project that daunt me.

Sadly that wouldn't be strictly true because most jobs that don't simply involve destruction or easy mind-numbing repetition daunt the hell out of me and I usually find a reason to fear it and avoid it for as long as possible.

The old mortar was cracked and failing so
I chiselled it out to a depth of around 10mm or so
One of those jobs was re-pointing the Living Room chimney breast with lime mortar.

The first parts of the job were easy repetition and destruction - chiselling out the old lime pointing, which had been damaged by the mechanical removal of previous coatings and blasting, and cleaning up the faces with brick acid, which so far has been one of my favourite consumable DIY products of all time, along with Gorilla Glue, Frog Tape and disposable pencils from hardware stores.
But I had been procrastinating over the actual re-pointing job because my one and only previous attempt had been on the front wall where we put the new house sign, and I needed it done in a rush. It was (and still is) a bit of a mess because I hadn't really read-up on how to do it and I had used some left-over mortar from someone's else's job up on the roof. I had never even mixed-up mortar - of any kind - before.
So to do a job that could result in an ever-lasting mess that made me hate my clumsy hooves and over-ambition was, you know... daunting.

That was one of the reasons we wanted to go on the lime course at Ty Mawr, where Dawn could pay attention to the details and get to grips with some practical stuff while I, as Primary DIY Monkey, would get annoyed at my inability to learn any new skills absolutely perfectly and absolutely immediately.
The plan worked and back home we put our heads together and came up with a plan: I just man-up and get on with it.

There are a few ways to knock-up (that's the actual technical term, which I now use in casual conversation to make it sound like I know what I'm on about) a lime mortar mix but the two most cost-effective ways, without getting your hands on an expensive cement mixer for large jobs, is to do it in small batches by hand with a spade or trowel - which is pretty hard work and difficult to mix thoroughly, or to buy an electric paddle mixer and whisk-up a slightly larger batch in a big flexi bucket. I did the latter because power tools are power tools at the end of the day, and I'm a bloke.
Incidentally, if you just happen to be doing the same type of thing and end up mixing too much mortar, just tightly cover the left-overs with a plastic bag and it should be okay for a day or two. If it starts drying out you can bring it back to life with a little bit more water (less is more) and a mix. It'll keep indefinitely that way, unlike cement.

Being a bit too generous with the water the first couple
of times, this mix is a bit wet but it was still okay
Using an empty 1kg lime putty pot, which was replaced by a child's sandcastle bucket when it split, I measured out a 12:4 (3:1) mix of yellow sharp sand (red sand makes the mix pinkish and I wanted to keep it as white as possible) and NHL3.5 lime from a black bag which someone Dawn knows donated from his garage a few months ago. I mixed everything up dry with the paddle mixer first then slowly added water from a watering can, paddling as I went.

Knocked-up lime mortar should be quite a dry consistency to reduce the chances of cracking as it dries and cures, but just a single splash of water too much in the mix makes wobbly soup so it took a little while to get it right. Also, as I was mixing, I realised that for some strange reason the lime powder hadn't kept too well in its bin bag in the damp outbuilding and some of it was holding together in little 'rocks' so I crumbled the bigger ones up as I went, remembering that all of the original lime pointing I had seen throughout the building held little lumps of pure lime where much the same thing would have happened back in The Old Days. So at least it was accidentally authentic.

The process of pointing-up itself, which was the part I was dreading most, was actually pretty simple and quite satisfying when all was said and done.

It looks like a sloppy mess but 'knocking it back'
with a scrubbing brush covers a multitude of sins
Having decided to use a narrow pointing trowel (which sounds bleeding bloody obvious, but other types of small trowels can be used with equal success) and a dinky 20cm-sq hawk from Ty Mawr I simply watered the area I was working in with a garden sprayer (which slows the drying process and reduces cracking/shrinking), slopped a little of the mix on the hawk, offered it up to the chiselled-out pointing and pushed morsel after morsel in to the gaps with the trowel, trying to be as neat and careful as possible to keep the brick faces clean.
As you go along, the trick here is to push too much mortar in there so the new stuff stands a couple of mm proud of the brick faces around it, so you can 'knock it back' later on.

As I got lower towards the mantelpiece (which was covered) some of the bricks, which had already been a bit loose and wobbly before I started chiselling, had decided to pretty much bounce around in the load-bearing structure which was somewhat worrisome, so I packed (I think 'pinned' is the proper term) around the loose ones with slivers of slate and stone to stabilise them then made sure I pushed the new mortar in hard to fill any gaps.

Finally, a couple of hours after I had pointed an area, and as the mortar was beginning to go off - but before it dried too much - I went across it gently with a stiff-bristled hand-held scrubbing brush, tap-tap-tapping at the raised pointing and 'knocking it back' until it was flush with the brick faces. This way it softens up any hard edges from the trowel, blends the junctions between the horizontal and vertical lines, jettisons tiny pieces of aggregate from the mortar's surface to makes it tactile and more interesting, increases the mortar's surface area to help the structure breathe and generally tidies everything up as it knocks overenthusiasm off the edges of the adjoining brick faces. 

A couple of weeks later, once I knew that the mortar would have thoroughly dried and cured, I took a small paintbrush and some more brick acid to the faces for the final tidy-up, then once the mantelpiece was uncovered and resanded (down to 240 grit) the stove door was cleaned, we maintenance-waxed and refitted the companion set and had a general tidy-up. This is the final result:



Incidentally, because I'm planning a few posts in fairly quick succession you might have missed that we've now moved in to the house!

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