Repointing - Oh God I am confused

[biffvernon]

Sandstone is, essentially, either grains of quarts bound together by a quarts matrix ( very hard rocks, difficult to work, so not often used in building) or gains of quartz bound by a calcite matrix. In such a sandstone, if the matrix has dissolved out, probably mostly due to carbonic acid in the rain, then reintroducing calcite would arrest and reverse the process of wethering. Soaking the porous rock with limewater seems a good way of achieving this. I guess it's effectiveness depends on how much lime gets introduced. There has to be enough for a new calcite matrix to bind the quartz grains and this may involve a great many repeated soakings of limewater.

 

[Penners]

Aah - sounds like the building equivalent of curing colds with echinacea.

 

[MdB]

Sandstone is, essentially, either grains of quarts bound together by a quarts matrix ( very hard rocks, difficult to work, so not often used in building) or gains of quartz bound by a calcite matrix. In such a sandstone, if the matrix has dissolved out, probably mostly due to carbonic acid in the rain, then reintroducing calcite would arrest and reverse the process of wethering. Soaking the porous rock with limewater seems a good way of achieving this. I guess it's effectiveness depends on how much lime gets introduced. There has to be enough for a new calcite matrix to bind the quartz grains and this may involve a great many repeated soakings of limewater.

That was my initial thought too. However if you read this http://www.induni.net/page24.html they suggest that the binding can just occur on the outside layer which can then flake off as a sheet. I might still give it a go an a spare piece of stone I have (very weak crumbly sandstone).

 

[Gareth Hughes]

I think, if limewatering actually does anything, that it adds a layer of lime to the surface of the stone, which will also soak a short distance into the stone and fill small pores, fissures etc, giving (like those face creams full of gamma-hydroxy-madeupanyl) a “tighter” appearance and hardening the surface. Whether this is a good idea, or of any long-term benefit, is a debatable point. Painstakingly forming a crust of glorified kettle fur isn’t, though, the same as consolidating the stone as a whole and restoring some of its original strength, which is the claim made for the process by its most ardent adherents. It may be, as has been said above, that creating a crust with different characteristics to the underlying stone (of which it should be a consistent part) will work in the short term but lead to bigger losses when failure eventually occurs.

The late Martin Caroe taught part of the churches section of my conservation course at Plymouth – never had any reason to be anything other than grateful for his advice, and I was baptised in an excellent church designed by his grandfather. On the other hand, I did a quinquennial of one of his father’s churches in Reading which was is such poor condition it had to be demolished (the apse had slid off the bitumen DPC because thermal expansion was being contained by the chancel arch)

 

[Jon Maine]

Nothing worse than an apse suffering from expansion!!

I have to say in all seriousness whilst a stone conservator once wrote a report on my windows which included a lot of limewatering we never did any as it felt like a temporary fix anyhow. I was more interested in making the building as maintenance free as possible for as long as possible. It struck me that this was a technique which may arrest decay for a while but store up problems for later.

I took the philosophy if a stone was crumbly to the core throw it away, if it was crumbly only on the surface maybe a mortar repair to stop the water from holding, not to reshape to make like new was appropriate.

After all we would not leave a rotten piece of wood doing a structural job would we.

 

[biffvernon]

Those superficial comments make sense. Maybe one needs to soak the stone to saturation in limewater rather than just give it a quick wash. Tricky to do when part of a building.