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Posted: Thu Nov 22, 2012 3:37 am
by parksloper
We have just installed a 1" pennsylvania bluestone (type of US sandstone - feldspathic greywacke) patio with tiles butting closely - no grout between them. Stones were laid in cement on top of a recently laid cement slab. After a heavy rain, the contractor power washed the patio with water, to remove fresh acrylic paint that had run onto it due to an unexpected heavy storm. For the two weeks since then we have seen "window pane" stains around many of the tiles. and other darker areas on the tiles. The contractro says that these will go away with time when the tiles dry out. I am thinking that there is some residue that came out from the cement that is part of the discoloration and that this will not go away so easily. Should I ask the contractor to wash the tiles? if so, with what? Is it correct that the stones are taking a long time to dry because of all of the sources of water - the new cement underneath, the rain and the power wash? Thanks for your help!
Posted: Thu Nov 22, 2012 7:31 am
by lutonlagerlout
roger from R and A pressure washing is the man to answer this
almost certainly needs some kind of cleaning
LLL 
Posted: Thu Nov 22, 2012 8:34 am
by Tony McC
I think this is what we refer to as "picture framing" and it's usually a result of fine cementitious material being drawn into the stone from the joints by capillary action. I have to admit that it shuld be much less likely on unjointed stone, but because the side face of a joint offers an additional evaporation surface, it can (and obviously does!) happen as that 'cementy moisture' is drawn in via th base of the flagstone and then the water vented off via the open joint.
It will weather away, eventually, but it can be cleaned using an acid if you are exceptionally careful. Unlike many of the imported sandstones, Penn Bluestone is not overly susceptible to acid but it still needs care as some of the more colourful pieces can have an adverse reaction.
It must be the season for Penn Bluestone - I spent a couple of hours on skype last night talking about it with an important mid-west supplier.
Posted: Thu Nov 22, 2012 11:47 am
by London Stone Paving
you've got a real gripe with the imported sandstones Tony. I am not advocating using acid on any stone (always take professional advice before using acid on any type of natural stone) but I have personally used acid on all of the Indian sandstones that we import and have had no adverse effects.
The Indian limestones are a different story altogether and under no circumstances should acid be used on any of the Indian dolomitic limestones
Posted: Fri Nov 23, 2012 3:14 pm
by Tony McC
Steven: have you seen the photies on this page?
And that's just one case of several dozen that I've looked at over the last 5-6 years!
It doesn't happen every time and it doesn't happen with every type of imported sandstone, but it does happen and it happens enough for it to be a problem.
Posted: Fri Nov 23, 2012 11:27 pm
by London Stone Paving
There is no doubt that in the wrong hands acid can make a real mess of natural stone, regardless of the origin of the stone. You added in your post that even with blue stone which is less susceptible to acid, extreme care must still be taken. I would say that in the pictures you mentioned, the acid damage was more down to incorrect application of the acid than anything to do with the type of stone. Acid will damage any stone if not used carefully and that includes any of our British stones
Posted: Sat Nov 24, 2012 9:58 am
by Tony McC
In those photies, I know for a fact that the acid application was meticulous. I watched it being done! It was 1999 or 2000 and it was the first time I'd seen Kandala Grey being used. The supplier was certain that acid cleaning just to finish off the job couldn't do any harm, so we took his word for it.
Flags were pre-wetted, acid diluted to 3% and applied via a fine rose on a watering can, scrubbed with a PP stiff brush, given 10 minutes then rinsed off well before the surface had even thought about starting to dry.
It was a real shock 48 hrs later when the contractor asked me to go back to site with him because there had been no indication on the day of application that anything untoward was happening.
It was also the first time I noticed that the flags that had been in direct stong sunlight during application were much more affected than those in permanent shade.
Posted: Sat Nov 24, 2012 12:20 pm
by London Stone Paving
I've got an answer for everything Tony 
There is a certain kandla grey quarry where the quarried blocks have a very high iron content. The quarry is quite well known in India and the stone is now only used for the Indian domestic market. In the past though before the problem was discovered a lot of this stone was imported to the UK
Posted: Sat Nov 24, 2012 12:52 pm
by lutonlagerlout
i have used acid on modak and autumn blend with absolutely zero adverse effects
tried it on an off cut first though,day before
wouldn't fancy using it on mint though
LLL
Posted: Sat Nov 24, 2012 3:15 pm
by London Stone Paving
Used it on mint loads of times. Never had any problems.
Posted: Sat Nov 24, 2012 3:22 pm
by RAPressureWashing
parksloper wrote:We have just installed a 1" pennsylvania bluestone (type of US sandstone - feldspathic greywacke) patio with tiles butting closely - no grout between them. Stones were laid in cement on top of a recently laid cement slab. After a heavy rain, the contractor power washed the patio with water, to remove fresh acrylic paint that had run onto it due to an unexpected heavy storm. For the two weeks since then we have seen "window pane" stains around many of the tiles. and other darker areas on the tiles. The contractro says that these will go away with time when the tiles dry out. I am thinking that there is some residue that came out from the cement that is part of the discoloration and that this will not go away so easily. Should I ask the contractor to wash the tiles? if so, with what? Is it correct that the stones are taking a long time to dry because of all of the sources of water - the new cement underneath, the rain and the power wash? Thanks for your help!
Can you post a photo of the staining?
I also noticed you are in the USA NY, I do know some companies that I can put you in touch with, once I have seen what needs doing.
Posted: Sat Nov 24, 2012 7:48 pm
by Tony McC
I think rainwater can be a threat to Mint!
I had heard something about a 'rogue quarry' previously, but as I still occasionally see Kandla Grey being affected by acid cleaning, I assumed it was an alibi or excuse put out by dubious suppliers.
Some of it must be slipping through, Steven, because I've seen some acid-rusted Greys over the course of this summer. I suppose there'll always be a few charlatans in the stone supply trade, both here and overseas, and if they can pass off sub-standard stone at the same price as the good stuff, they will.
Posted: Sun Nov 25, 2012 5:02 pm
by London Stone Paving
Some material will always be slipping through. We had a batch 5 years ago that turned brown on its own, without the help of acid. We managed to fix the problem without ripping it up. I can't recall the name but we used some substance that draws out the iron content. There was also a project this year where some of the kandla grey turned brown. We couldn't fix it so we had to rip and relay. There has been a bit of shortage of kandla material this year and we found out that our suppliers had bought blocks from a different block supplier. It turned out that these blocks had come from this bad quarry. We obviously had to finance the cost of the rip, which was then passed onto our suppliers
Posted: Wed Nov 28, 2012 3:28 am
by parksloper
Thanks everyone. some water and a soft brush were used and after a few days of good sunlight, the whole thing looks better. the window pane effect seems to be gone now that we've had about 5 days of sun and no rain. the contractor says that the slab under the stones was still not quite dry, and with heavy rain and power washing really forcing water between the stones, they were slow to fully dry, leading to the window pane effect. So problem solved!
Posted: Wed Nov 28, 2012 7:47 am
by lutonlagerlout
another satisfied customer
LLL