Lime Mortar

[tony t]

Ladies and Gents,

New on this forum please be patient.

I have recently purchased a lime stone fire surround and the manufacturer informs me that lime mortar is to be used as standard mortar will stain.

Manufacture will give no further help.
Spoke to local builder who was fixing lintel at the fireplace.
He dropped of a bag of Hydrated lime.
Do i mix this with sand and use as a mortar or mix with water and make a putty.

Your help is much apprecited

Best Regards

Tony.

P.S
Would this mortar be suitable for pointingthe gap between the existing stonework and my new wooden sash and case windows rather than using silicone.

 

[biffvernon]

Mix the lime powder with sand in a ratio of about 1 part lime to 3 parts sand by volume. Mix thoroughly and then add as little water as possible to get a workable mortar. Make sure you wet the surfaces you are applying the mortar to and give the new work an occasional spray for a couple of days if you can. The more slowly it dries the better. It will go hard after a few days and gradually get stronger over a period of several weeks as it absorbs CO2 from the air.

Some folk will say you should soak the lime in water for a few months and use the resulting putty and others will say you should buy lime putty to start with. But then some folk will say you should spend £10 on a bottle of wine rather than £5. I prefer to spend £5. Twice.

You can use lime mortar to fill the gap round the window and it will probably be just fine. But there is a risk that a little crack opens up between the wood and the mortar. Wood moves with the seasons and does not bond tightly to the relatively stiff lime mortar. The alternative is to make up a paste of sand and linseed oil. Use very fine sand - I sieve it through a kitchen sieve - and just enough boiled linseed oil to make a workable paste. It will, rather alarmingly, stay soft for a long time but then just when you've forgotten about it you will find that it has turned into a hard mortar-like material that sticks very firmly to the wood and remains just pliable enough to accommodate movement without cracking.

If you don't fancy mixing your own then buy'Burnt Sand Mastic', which has some rosin in the mix and will set a bit quicker. It's quite expensive so if the gap is big I would fill most of it with lime mortar first and then fill the remaining small crack with sand/oil mastic.

It will last forever. Silicone won't.

 

[NT]

The usual way t make lime mortar is to mix hydrated lime with water to make a paste, store it airtight for a few weeks, then mix 3:1 with sand and use. 3 sand: 1 lime

NT

 

[tony t]

Many thanks for the replies,

After posting the initial question, i found out how to search (a bit of a computer novice, but learning) and found outabout burnt sand and linseed oil,

I did think about making my own, why do the manufacturers heat the sand,is this required.

I am trying to rebuild myvictorian home slowly but surely in the correct manner and rip out all the damaged upvc etc and many pther horrors.


Excellent Forum


Best Regards

Tony

 

[NT]

tony toshney:

>After posting the initial question, i found out how to search (a bit of a computer novice, but learning) and found outabout burnt sand and linseed oil,
>
>I did think about making my own, why do the manufacturers heat the sand,is this required.

they wouldnt do it if it wasnt. Another homemade mastic is boiled linseed and chalk.


NT

 

[biffvernon]

they wouldnt do it if it wasnt. Another homemade mastic is boiled linseed and chalk.NT

I've asked around but not had a convincing answer to the question of Why burn the sand? Something to do with the colour is a common view. Of course sand doesn't actually burn - you'd have to heat quartz to about 1400 degrees before anything happened, and then it would melt and you wouldn't want that. So the 'burning' must be affecting the tiny proportion of the sand that is not quartz in some mysterious way. I've never bothered but maybe I'm missing something.

Linseed and chalk is what ordinary putty is made of. I've heard that mastic is made by mixing the oil with sand and chalk so I guess there's a continuous gradation between the fine putty used for glazing and the coarse mastic used for setting window frames. Sand is cheaper, bulkier less liable to crack?

 

[NT]

Sand contains a percentage of shell material, burning it converts it to the evil form of lime.

NT

 

[biffvernon]

Hmmm, maybe. But the calcium carbonate to calcium oxide reaction sets off at getting on for 9oo degrees. I doubt if that temperature is reached when sand is 'burnt' on a hotplate. If a bit of quicklime helped, surely it would have been easier to add some lime from the local kiln?

 

[NT]

> mmm, maybe. But the calcium carbonate to calcium oxide reaction sets off at getting on for 9oo degrees. I doubt if that temperature is reached when sand is 'burnt' on a hotplate.

is that how its done? how hot is a red hot hotplate?

> If a bit of quicklime helped, surely it would have been easier to add some lime from the local kiln?

Maybe so. Burnt sand vs burnt limestone... ?


NT

 

[ste-pee]

erm, does burning the sand simply drive off any residual moisture, allowing the oil to mix properly?
-just a guess.

 

[biffvernon]

That's what I've thought.