The Brew Cabin

Posted: **Thu Jun 18, 2020 8:48 am**

Hi, hoping to get some help on an issue I'm having.
I have a strange purple stain pattern starting to appear on my sandstone patio, any idea what it could be and what to do about it?

It's strange as it's only on some flags and not on the ones directly beside them, seems like something in those particular flags is reacting with an external element producing these stains

The only thing that comes to mind that might had a knock on effect is that we have a grease trap which was full, this resulted in the drain getting blocked and everything from the sink and dishwasher was running out onto the patio for a couple of days.

I've tried to scrub it with some bleach and wire brush brush but had no effect at all, power washing didn't do anything either

I am looking to apply advanced-HD sealer this summer so would like to get rid of these stains in advance if possible

link to image

Posted: **Thu Jun 18, 2020 11:22 am**

I think that's a mineralisation effect, a change in the mineralogy of the stone on exposure to air and damp.

The only other thing that springs to mind is the red algae that is prevalent in Ireland, but, were it that, I would expect to see it affecting all (or at least most) of the flags, not just specific ones.

I'd hold off the sealant just yet. Let this process continue and see what happens next....sometimes, these mineralogical/petrological changes in colour play out over a few months and then settle down to a fixed state, with no further change.

Posted: **Thu Jun 18, 2020 12:59 pm**

i think your right about the mineralisation, think algae would spread in a more territory consuming manner
I'll keep an eye on it for next few months see how it changes, wouldn't really mind if it just settled on purple instead of being patchy
BTW this patio is down about a year now, would have thought these processes would have already taken place by now( i was waiting the year for the efflorescence to weather off before sealing)
I also spotted these on few other flags that are on the other end of the house though much less noticeable

Posted: **Sat Jun 20, 2020 11:17 am**

The thing about mineralisation is that it can happen almost immediately or it can be a long slow process....all depends on the stone and the minerals involved.

At least you have some reference photies that you can 'compare and contrast' in a year's time.

The Brew Cabin

Purple algae like stains on sandstone