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Acid cleaning: results
  
  Index -> Conserving, Preparing and Cleaning Minerals
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Rei




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PostPosted: May 07, 2014 17:04    Post subject: Acid cleaning: results  

So, here's the results from my different acid cleaning attempts of chalcedony. Chalcedony, as a crack-filling material, naturally tends to come as "plates" with a crust on each of their long sides: you only see the chalcedony itself on the thinner, broken edges, and there are sometimes ugly deposits on them (iron oxide, etc). To make matters worse, chalcedony can be brittle, which complicates aggressive mechanical cleaning. My goal was to see what I could remove with acid cleaning

Citric acid: I first tried a bunch of random home chemical products - citric acid, peroxide, bleach, vinegar, and acetone. Of these, only citric acid had a noticeable effect. But the effect was rather weird, as you'll see in the picture, it ate out some unusual shapes on the surface, but not the surface as a whole.

Oxalic acid: Very disappointed with this one, as it's often the top recommended acid cleaning for quartz-based minerals. Yeah, it removes surface films, but as you'll see, everything it doesn't remove changes to ugly colors. I've just started a new series to tests to see whether hydrochloric can remove what oxalic left behind. Oxalic is fairly strong and is toxic, but I found it in general pretty easy to work with. I had it at 5% strength.

Hydrochloric acid: Very pleased in general with this one. You'll see that it generally cleaned things quite well. What it did leave behind (what appears to be the silica-based components of the crust) isn't unattractive and is easier to remove than the crust itself (though still not "easy"). It even bleached the "gunky" look from my test "ugly" chalcedony specimens, giving them a light appearance that lets their natural color shine through, specially when wet. The only gotcha is that, of course, 30% HCL is nasty stuff. I have to hold my breath when working near it (walking away to breathe) (a simple dust mask won't have any affect, it'd take a true gas mask), and the stuff is like liquid fire, I wear a full body work suit, rubber gloves, eye protection, etc.

Oxalic / hydrochloric acid together: Still testing this one, but I think it's going well. I ran into a research paper on cleaning with acids (designed for cleaning metals, but still) that mentioned that oxalic is sometimes added to HCl to reduce outgassing when cleaning. I figured that they might work well together - HCl for its acidity, oxalic for its mineral-binding affinity. So far, the results look fairly similar to HCl alone. I'd probably in the future just use HCl, but I happened to have the leftover oxalic on hand.

I'm still trying to get phosphoric acid to try out, as that's another one reported to be good at cleaning. I could potentially try sulfuric, as it's another strong acid, but I would imagine the sulfates would tend to discolor. Nitric is yet another strong acid, but I'd rather not, people would probably think I was up to no good ;)

With all strong acids, I found it important to keep a bucket of clean water nearby at all times - not just for emergencies, but simply to - for example, rinse off tongs after manipulating rocks in the acid. They come out dripping with acid, so of course you need to clean them off. I also kept a box of baking soda around as a precaution.

As with standard acid-treatment procedures, all samples removed from the acid are being stored in regularly-changed water for long after their removal to prevent unsightly surface deposits. The acid works its way deep into the minerals and takes just as long to work its way out, and it's full of dissolved minerals which will deposit on your mineral if they simply dry out.



s1390034.jpg
 Description:
Chalcedony / citric acid
Hvalfjörður, Iceland
 Viewed:  11405 Time(s)

s1390034.jpg



s1390033.jpg
 Description:
Chalcedony / citric acid
Hvalfjörður, Iceland
 Viewed:  11388 Time(s)

s1390033.jpg



s1390003.jpg
 Description:
Jasper / oxalic acid
Hvalfjörður, Iceland
Note how the naturally-broken side cleaned up nicely - it only had a bit of film on it.
 Viewed:  11345 Time(s)

s1390003.jpg



s1390001.jpg
 Description:
Jasper / oxalic acid
Hvalfjörður, Iceland
... but the crust? It came out ugly.
 Viewed:  11379 Time(s)

s1390001.jpg



s1390063.jpg
 Description:
Chalcedony / oxalic acid
Hvalfjörður, Iceland
A disaster of a result from oxalic acid.
 Viewed:  11382 Time(s)

s1390063.jpg



s1390064.jpg
 Description:
Chalcedony / oxalic acid
Hvalfjörður, Iceland
That's not actually blue chalcedony, it's brown. The crust became blue here.
 Viewed:  11405 Time(s)

s1390064.jpg



s1390065.jpg
 Description:
Chalcedony / oxalic acid
Hvalfjörður, Iceland
 Viewed:  11359 Time(s)

s1390065.jpg



s1390067.jpg
 Description:
Chalcedony / oxalic acid
Hvalfjörður, Iceland
 Viewed:  11389 Time(s)

s1390067.jpg



s1390045.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
 Viewed:  11400 Time(s)

s1390045.jpg



s1390046.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
 Viewed:  11360 Time(s)

s1390046.jpg



s1390047.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
 Viewed:  11378 Time(s)

s1390047.jpg



s1390048.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
 Viewed:  11370 Time(s)

s1390048.jpg



s1390049.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
This was a test of putting a "junk" piece in the HCl and seeing what happens. By "junk" I mean a piece where there's evidence of chalcedony but its fragmented and mixed with more basalt than chalcedony.
 Viewed:  11406 Time(s)

s1390049.jpg



s1390051.jpg
 Description:
Chalcedony
Hvalfjörður, Iceland
 Viewed:  11343 Time(s)

s1390051.jpg



s1390054.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
Note how the crust that remains isn't ugly and discolored like with oxalic, it's just eaten away in natural-looking manner.
 Viewed:  11369 Time(s)

s1390054.jpg


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Rei




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PostPosted: May 07, 2014 17:13    Post subject: Re: Acid cleaning: results  

A few more pics


s1390027.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
Example of the results on a different kind of "junk" piece - a piece that's dark, dull, dirt-colored, not as glassy... basically inferior chalcedony. After acid treatment, note how light it is. It's also become a bit porous as the contaminant material was eaten away.
 Viewed:  11371 Time(s)

s1390027.jpg



s1390028.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
 Viewed:  11383 Time(s)

s1390028.jpg



s1390031.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
It looks much better when wet, though. A good waxing or polyurethane coating it could keep that look indefinitely.
 Viewed:  11339 Time(s)

s1390031.jpg



s1390032.jpg
 Description:
Chalcedony / hydrochloric acid
Hvalfjörður, Iceland
 Viewed:  11342 Time(s)

s1390032.jpg


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Mike Wood




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PostPosted: May 09, 2014 15:05    Post subject: Re: Acid cleaning: results  

Hi Rei, thanks for posting all the photo's of your results: quite the alchemist, aren't you?!
The photo's are interesting and informative for those of us who aren't used to doing this kind of stuff.

" I first tried a bunch of random home chemical products - citric acid, peroxide, bleach, vinegar, and acetone."
"Nitric is yet another strong acid, but I'd rather not, people would probably think I was up to no good ;) "
- I wouldn't have thought, after mentioning peroxide and acetone in the same sentence, that nitric acid would attract any more attention than you (and now me) might have done already!!
Seriously (again) though, nitric acid is pretty nasty stuff. Usually evolves nitrogen dioxide when it's in action dissolving things, and you definitely don't want to breathe that in.
Sulphuric acid is also very nasty. It has the tendency to become more concentrated (if you spill any) as it evaporates. Also can start a fire when in contact with flammable material (eg a rag used to wipe up spills) because it is a very powerful oxidising agent.

Careful now! (As Dougal would say).

Mike

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Rei




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PostPosted: May 10, 2014 02:44    Post subject: Re: Acid cleaning: results  

Heh, your average person uses citric acid, vinegar, peroxide, bleach, and acetone in daily use. But I doubt more than 5% of people have ever owned concentrated nitric or sulfuric acids ;) And yeah, I know they're nasty for other reasons. HCl is pretty nasty too, but I take precautions - eyeglasses, full body suit, rubber wading boots, half-arm rubber gloves, water and baking soda adjacent, etc, and the most important, not breathing anywhere near the stuff when the bucket is open ;) I've got on order a sandblasting / painting helmet with air filter and full-arm gloves.

Concerning nitric, though, I just know that two of the biggest problems police fight against - terrorism/bomb making and crystal meth - both involve the use of nitric acid. So if I went out looking for liters of the stuff out of the blue, I would expect to end up on several watch lists ;) Also, nitric is one of those acids where if you mess up, it's not that unlikely that you accidentally make something that *explodes*.
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