Friday, 13 May 2016

The use of lime putty.



It’s been a while since I had a rant about the use of lime in comparison to cement but as Doris and I aren’t doing anything today I may as well pen a little invective.

The trouble with the building trade is that it’s very slow to change. Mention continuous professional development to anyone in construction and apart from those who wear the white collars you’ll get a dumb look. The reason for this is because unless a product will speed up a process or coin a larger profit then people will not use it. In fact, you’ll have to drag them kicking and screaming if you want them to use traditional materials  or natural products.

In the case of lime it is shunned because it takes more time and skill than modern products therefore it is regarded as something which will cost more to use and therefore affect competitiveness where quotations are concerned. Cement is where it’s at - unfortunately ease of use is promulgated in the construction sector to the detriment of the built environment.

Nevertheless, this is not where members of the general public are. People have moved on. Home owners are savvy. They research. They hear things on the grapevine and get themselves up to speed. Only 10 years ago I was struggling to inform would be clients that lime was necessary on their old houses. Not anymore. Thanks to the internet, pretty much everyone who contacts me has a basic understanding of the mechanics of older properties. All I need to do is help them out where a bit of practical knowledge is concerned and maybe quell fears they have regarding appropriate conditions for use.

The problem many people in the English speaking world now have is that when they give the average builder the opportunity to build or repoint using lime, the first thing he does is knock up a bit of lime mortar with far too much sand and then add a bit of cement. Ask him why and he’ll tell you “it sends it off quicker”. In his eyes the addition of cement is needed for a quicker set. He'll also believe that the lime makes his mortar more porous so that’s got to be better than just cement on its own. In his world, that’s lime mortar. It’ll also last longer because it’s harder.

Wrong. 

You’ll get about 40 years out of cement repointing and during this time all manner of damage to your stone will be being done. In direct contrast you can expect between 70 and 200 years out of lime depending on type of stone and amount of weathering. Even today I'm still working on buildings with the original lime still in their joints. Time is an excellent measure of durability. 

If you want to use a landmark as a comparison for longevity then try the Tower of London. That was built in 1078. If you want another example there’s always Hadrian’s Wall.

The Slovenians had the right idea and instantly banned Portland Cement. In fact, it’s only really been the English speaking world which embraced it. If you want proof, try telling an Italian stonemason in the heart of Puglia that he’d be better off with a bag of Rugby general purpose and see how far you get. Although he will be quite used to seeing failing mortar joints, they’ll be degrading on buildings which were originally built in places like Marziolla around 1550. 

In direct contrast we have Bob the Builder. I take great delight and merriment from reading posts on self-help “ask a builder” websites where builders and anyone with an interest in construction are quite happy to proffer their expert advice. The last one I read was where someone wanted to use lime on an elevation of their property which experienced buffeting from the weather. One guy suggested using NHL 5 half way up the elevation and 3.5 for the rest. I was amazed. Not only would there be a colour difference but an NHL 5 is like using a sledge hammer to crack a nut. Totally unnecessary.

Oh, and if you want a laugh: I actually heard of one guy recommending the usual cement/lime mix on aged Cotswold ironstone. The reason? To stop the rain washing out the mortar.

I was astonished..

But I digress. When I first started working with lime it was always natural hydraulic lime (3.5). It took years to understand its temperament and even today I occasionally get caught out. But I always like to keep myself challenged so when the opportunity to use putty presented itself I couldn’t resist.

I think it’s partly to do with the fact that it’s older than NHL and less cementitious. It harks right back to Roman and Egyptian times and is what the masons and builders would have used all over the world prior to Smeaton inventing natural hydraulic lime back in the mid 18th century. Putty is a non-hydraulic lime and in essence this means it won’t set underwater and won’t cure if applied to damp areas. In direct contrast, hydraulic lime will set in the presence of moisture and is slightly different (and easier) to work with. Alongside many variables to deliberate one needs to carefully consider the amount of care which needs to be taken when mixing. If, for example, one uses wet sand then all that’ll happen is your mortar will turn to slop. But carefully planned, the use of putty can be a rewarding experience both for contractor and client.

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