Stained Slate

[Regent Dave]

I have stripped a Victorian fire surround back to bare slate, using alkaline poultice stripper.

I realise that it would originally have been painted, but the dozen or so coats of paint had completely obscured all the detail. Thus it was the only option.

The bare slate is disfigured by approximately half a dozen small circular marks (various sizes, up to a maximum of approx 1cm diameter).

The marks are greenish in colour & resemble the colour of oxidised copper.

They appear to have formed around small "nicks" in the surface of the slate that have allowed the discolouring substance to penetrate the slate.

I suspect that the original layer of paint was the root cause of this. It was a combination of black with red inlays & appeared to be some sort of lacquer or enamel.

Does anybody know what has caused the marks?
Or how I can remove / disguise / lessen the appearance (presumably by neutalising whatever chemical was in the original coating)?

I realise that re-painting it would solve the problem, but now that I've buffed the bare slate it looks absolutely majestic, so I'd like to leave it as such. Not strictly Victorian, but my preferred option nonetheless.

A very similar question was posted back in October 2005 & received no replies, so any suggestions would be appreciated.

Thanks

 

[Lime]

The bare slate is disfigured by approximately half a dozen small circular marks (various sizes, up to a maximum of approx 1cm diameter).

The marks are greenish in colour & resemble the colour of oxidised copper.

They appear to have formed around small "nicks" in the surface of the slate that have allowed the discolouring substance to penetrate the slate.

I suspect that the original layer of paint was the root cause of this. It was a combination of black with red inlays & appeared to be some sort of lacquer or enamel.

Does anybody know what has caused the marks?
Or how I can remove / disguise / lessen the appearance (presumably by neutalising whatever chemical was in the original coating)?

Perhaps the marks are the remnants of ornaments, figures or a clock with metal feet?
Have you tried metal polish, T-cut or stoveblack to remove or cover the marks?

I have been told that the original paint on the fire surround would have been applied in a water bath. The paint was floated on the water and the slate raised up through it to achieve the marbling effect. Perhaps other more knowledgable posters could confirm or deny this process.

 

[Regent Dave]

The marks are on the vertical, not horizontal surfaces. Thus we can rule out ornaments with metal feet.

Is "stoveblack" the same as the grate polish I use on my cast iron fire surrounds? Or is it a reference to the coating that is used on wood burning stoves?

I hadn't considered T-Cut or metal polish.

I have used "Slate Sheen", which cleaned the slate very well & in the process highlighted these green marks.

 

[Lime]

The marks are on the vertical, not horizontal surfaces. Thus we can rule out ornaments with metal feet.

Is "stoveblack" the same as the grate polish I use on my cast iron fire surrounds? Or is it a reference to the coating that is used on wood burning stoves?

I hadn't considered T-Cut or metal polish.

I have used "Slate Sheen", which cleaned the slate very well & in the process highlighted these green marks.

The stoveblack is the same as you use on your grate.
Perhaps the marks have been made by a red hot poker or some sort of fire guard. Whatever caused them I have found that metal polish or T-cut works well on slate french clock cases to remove any scratches. It does take a lot of elbow grease though!

 

[NT]

The marks are on the vertical, not horizontal surfaces. Thus we can rule out ornaments with metal feet.

Maybe one of the ornaments was clawing at the slate?


Verdigris is mobilised by vinegar, but I dont know what effect if any that has on slate.

I certainly wouldnt repaint it. Imperfect and majestic beats perfectly dull any day.


NT

 

[plumbers mate]

Are you sure it's not a coloured bit of slate - depending on the quarry it can have green, sometimes very pale green, "blobs" of colour in it.
In which case it'll never go.

 

[Regent Dave]

It's not coloured slate.

I've tried to insert an image, but without success.

Does the image have to be hosted at a separate url?
Is it not possible to insert an image from my hardrive?

 

[Regent Dave]

Thanks to the advice posted by Nemesis, I've managed to upload a photo:

[/url][/img]

 

[Lime]

The picture appears to show the mark has an indentation in it and that it is very much part of the slate.
I'm not sure that the mark will be able to be removed.
Perhaps cosmetic treatment would be more effective.

 

[plumbers mate]

That picture certainly looks like a piece of slate from the Nantlle Valley area probably. The colour is right and the "stain" is right. You can pick up a million pieces like it around the Dorothea/Talysarn area.
It's been in that piece of slate a canny long while and nothing will shift it - ever.

 

[Regent Dave]

Wow! You can tell where it's from just from that small photo - I'm impressed.

As it's natural discolouration, I'm quite happy to leave it as it is.

I thought it must have been some sort of reaction to the coating because at the centre of every green mark there is a small "nick" in the slate surface & every "nick" has a green mark around it.

Plus, the marks seem to be very regular shapes (perfect elipses). I'd naively assumed that naturally occurring pigmentation would be more irregularly shaped.


Does anybody have any idea what causes these marks to occur during the formation of the slate?

 

[citizensmith]

As a geologist - i recon these look like some sort of "reduction spot" could they be spherical if you think in 3d. Do any seem to have a darker spot in the centre? If so then this would be a spot bof organic matter that had an oxygen reducing effect in a zone around it.
I imagine that if they were intended to be painted then it was OK to use a slightly inferior slate from the quarry. It may have been the only way to get large enough bits that did not split so readily like roofing slates.

 

[Regent Dave]

Yes, they very probably were spherical in 3D.

Thanks

 

[biffvernon]

As another geologist - I don't see how they could be spheres. A section across a sphere gives a circle. But if it were an ellipsoid (kind of rugby ball-ish) then a section gives an ellipse. But hundreds of millions of years ago it would have been a sphere. The rock was then compressed, deforming the sphere to an ellipsoid and metamorphosing the sediment to slate. By measuring the difference in the lengths of the three axes of the ellipsoid one can assess the degree of deformation and hence the movements of continents in the Ordovician Period, which was quite a long time ago. My old tutor, D. Flinn, wrote the definitive paper on this subject in 1956.

 

[middi]

No wonder why you stopped collecting certificates a long time ago Biff, somehow I get the feeling there isn't anymore certificates that would do you any good.

A geologist as well now. Unfortunately my first hand knowledge on the geological front extends to the strata of the earth crust to about 2 feet deep, just enough to put fence posts in.

I wish i could offer a learned comment on the marble inperfection, but my pennyworth tells me that if took a couple of hundred million years to sod it up it should take at least just as long to reverse the process.

 

[citizensmith]

I stand corrected by Biff. They should indeed be deformed spheres, presumably compressed parralel to the cleavage that forms the slates apparent layering.
I am most impressed with your bredth of knowledge! I blame the school holidays for making my brain go soft. Still - back to school next week!

 

[Moo]

And you will try to pay more attention in spelling lessons, won't you?

 

[citizensmith]

I know, I knew it when i was typing it - please feel free to hold me personally responsible for the decline in educational standards and the general state of the nation etc.

 

[Nemesis]

Oh, not personally - I'm sure a number of us can share the blame.

I blame the parents too however.

I gather that downloading a google toolbar gives you a spellcheck possibly, but I gave up caring a very long time ago.

 

[biffvernon]

They should indeed be deformed spheres, presumably compressed parralel to the cleavage that forms the slates apparent layering.

The original work was reported in:
Flinn, D., On the Deformation of the Funzie Conglomerate, Shetland. J.Geol. 64, 480-605: (1956)
and generalised in:
Flinn, D., On Folding during Three-dimensional Progressive Deformation, Quart. J. Geol. Soc., 118: 385-433 (1962).

Just in case you wanted to know that.