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 Post subject: working with honduran opal
PostPosted: Thu Sep 19, 2019 12:25 pm 
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Hi, my name is Tiago and im a geology student from Portugal. I've got a neat honduran basalt opal which i'd like to work and make a necklace. I read that this kind of opal won't polish that well but i'd like to try, maybe i can get a nice pendent out of it. I have no experience in lapidary since this is my first stone. I plan to follow this article: http://pacificopals.com/wp/opal-stores/ ... tructions/ . You guys think i can get a nice stone? Another thing is, i can't get opticon resin since i can't find it in Portugal nor i can buy it online. Could i use any type of resin?




the video i made: https://www.youtube.com/watch?v=8a_sj6j ... UqM3vpzIao

Thank you all !


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 Post subject: Re: working with honduran opal
PostPosted: Fri Sep 20, 2019 10:46 am 
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Hi all !

i want to ask a few more questions:

In this video-
,
SBMalls says that they used a process to carbonize the matrix, i think it's the same process they do on Andamooka opals, should i try it?

In the Pacificopals website they give the following tip: "After polishing the matrix swipe it with a black felt tip marker and rub the surface vigorously with a cotton towel. This will darken all of the tiny surface pits where the polishing compound is embedded." Does anyone already tried this method? Any feedback?

Thank you all !!


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 Post subject: Re: working with honduran opal
PostPosted: Fri Sep 20, 2019 10:00 pm 
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Hi Tiago, I'm wondering do you have any equipment to cut and polish the stone?
I've cut Honduran opal and it came out nice but not highly polished.
I haven't tried carburizing or a black felt marker but mine looked nice without either.
I'd be reluctant to try a procedure with which I wasn't familiar unless you don't mind potentially ruining your stone.

Have a good one, Jim


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 Post subject: Re: working with honduran opal
PostPosted: Sat Sep 21, 2019 2:01 am 
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It is fun material. Not actually basalt, but as far as I can tell some sort of more felsic rock that looks darker with the opal filling.
Heavily opalized areas will take a polish and look nice without treatment. The more porous stuff does seem like a good candidate for resin filling.
I would not start out with this material as my first lapidary experience. There are much easier materials to cut and polish that don't require treatment (which is a whole other set of skills and equipment).

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 Post subject: Re: working with honduran opal
PostPosted: Sun Sep 22, 2019 1:10 pm 
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Hi, thank you both for the replies. I think i don't have much to lose, i have access to concentrate sulphuric acid so i'd like to try the carbonization method, i have some 20.000 grit diamond powder for the pre-polish and use the cerium oxide for the final polish. I read that the cerium oxide is much fine than diamond podwer and can reach 100.000-150.000 grit and the problem about this kind of dark/porous matrix is that cerium leaves some white spots in the pores, but cerium oxide is soluble in mineral acids and i have access to nitric, sulphuric and hydrocloridric acid in really high concentrations so i could dissolve the tiny cerium oxide spots. I don't have the fancy lapidary machines but i just want to work this stone (and maybe one more for a special girl) therefore i think this will be a nice experience plus i like challenges.

Stephen, from my experience in the igneous petrology classes this is definitely a basalt, but has some kind of white powder around, but i can be wrong.

I think my biggest challenge will be the epoxy impregnation, do you guys think i can use any epoxy resin? Leave in the resin for some days, what you guys think?

Thank you !


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 Post subject: Re: working with honduran opal
PostPosted: Sun Sep 22, 2019 3:44 pm 
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I thought it looked and felt a lot like basalt in the hand when I first saw it too so I didn't question the ID, but here is a photomicrograph I took of a polished stone:
Image
Up close it's a bit of a different animal. I haven't looked at a thin section though.
I'm not sure how necessary the sugar treatment will be, vs just epoxy impregnation. If you can't get opticon maybe epoxy 330 would be a good choice?
I'm also not sure what strong acids might do to this material. I would suspect you might etch off more than just the cerium. In some cases a wad of mineral sticky tack pressed against the surface repeatedly will grab and remove contaminants, so I would definitely try that before acid cleaning.
Note that cerium isn't finer than diamond, it actually polishes with some chemical action rather than by pure abrasion. I don't think you'd see a great benefit from it on this material--cerium is great on silicates and glass, but the surface of this stuff is kind of inherently rough so you probably wont' see a difference from 20,000 diamond grit.

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 Post subject: Re: working with honduran opal
PostPosted: Sun Sep 22, 2019 5:36 pm 
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Hi again,
Nice photo!! Do you know the scale? If i had enough stone i'd make a thin section but for what i can see with my loupe, it's mainly anorthite with some pyroxenes/amphiboles and a few crystal i can't identify. Maybe in the future i will get more of this basalt and make a thin section.

i will try the carbonization method since i don't think the acid will harm the rock, i don't expect carbonates but i don't know if any of the inosillicates will react with the acid but it's a risk i don't mind to take (now the thin section would come in handy).

That sticky tack idea it's really cleaver i will definitly try it before the acid. About the cerium vs diamond, i read that the cerium it's much finer than the diamond which made sense to me but what you've said makes more sense, do you know where i could find more information about the reactions envolved in cerium oxide and polish? im just curious about the chemestry. About the epoxy, i will see what i can find in UK because in Portugal i will never find opticon products. But what makes opticon so especial? I read that the mollecules of their epoxy are smaller which doesn't make any sense to me since i think we are talking about the same mollecular polimerization of other epoxies, but im no way an expert about that epoxies, we use some kind of epoxy in thin sections but is expensive as hell and i cannot get any :( .

I will post every step i do, if i make a thin section i will record it and post it here :) (will take a while since im short on money and i have to pay for the section).

Right now im waiting for the dremel bits i ordered on Ebay to round some edges, then i will proceed to the carbonization. I have one question about the carbonization, i plan to use those electric stoves, but which temperature should i aim since im working with basalt, 150 ºC ? I plan to leave it on the stove for 3-4 days with some stirring each day + adding water each day but the sugar will burn, won't affect the carbonization process?

Thank you!


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 Post subject: Re: working with honduran opal
PostPosted: Sun Sep 22, 2019 8:02 pm 
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It certainly looks different from other basalts I have examined and I suspect it is something more felsic. I think Nathan Renfro thought it might be an ignimbrite. That all said, the world of volcanic/volcaniclastic rocks is messy and complicated and I've only done a bit of work with it. I don't have a personal petrographic scope so I haven't done any thin section work since I graduated but once I assemble one I'll probably have some made of promising rocks.

Just the sulfuric for carbonization should be fine, I meant that using acid to dissolve cerium out could potentially cause trouble. No guarantees on that though, just a hunch.
I don't know the exact chemistry involved with the cerium. That gets into some pretty specialized and potentially contentious stuff, and I am not certain it has been firmly nailed down. But chemical polishing definitely acts differently than abrasive polishing (one thing is that cerium oxide is softer than many things it polishes, and does not work well dry, needing some moisture).
Epoxy composition and details also edge into highly specialized and trade-secret-filled waters. So the exact why is always a bit mysterious for those of us outside, but opticon has a reputation for being reliable to use and lasting better than other options.
No idea on the temperature, as I haven't done this procedure myself. I am curious how it will go.

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 Post subject: Re: working with honduran opal
PostPosted: Mon Sep 23, 2019 11:31 am 
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TiagoPT wrote:
...do you know where i could find more information about the reactions envolved in cerium oxide and polish? im just curious about the chemestry.

This extract is from pages 25-26:
CERIUM
A GUIDE TO ITS ROLE IN CHEMICAL TECHNOLOGY
published by Molycorp, Inc.
Mountain Pass, CA U.S.A.
Library of Congress Catalog Card Number. 92-93444
(Cataloging-in-Publication Data)
Cerium : A Guide to its Role in Chemical Technology
Copyright ©1992, Molycorp, Inc.
(reprinted 1995)
Molycorp, Inc
67750 Bailey Road
Mountain Pass, CA 92366, U.S.A.
Printed in the United States of America

The most efficient polishing agent, by far, for most glass compositions (particularly those
produced commercially in large volume) is cerium oxide.[41] This application consumes, either as
a moderately pure oxide or as a cerium-oxide-dominated concentrate, a significant portion of the
cerium products produced annually. Commercial glass polishes are based on cerium oxide powders
with defined particle sizes and controlled dispersibility in aqueous systems.
Polishing is the act of producing a sufficient degree of surface smoothness so that light,
transmitted or reflected at that surface, is not disturbed by surface irregularities. The polishing
mechanism is still poorly understood at the chemical/molecular level. Polishing results in glass
removal and does show a dependence on chemical properties of the glass. The nature of the liquid
present during polishing is crucial and only if active hydroxyl groups are present, in alcohols for
example but especially in water, does the polishing phenomenon happen.
When a glass - typically an alkali silicate - is in contact with water, a complicated series of
steps take place, ion exchange, dissolution of glass constituents and possible structural changes. A
surface region of the glass is modified and it is this softer hydrated layer that is removed or
reformed during glass polishing. Classic abrasives produce an improvement in surface finish but
leave a fine but definite roughness, the scale of which relates to the grain size of abrasive used.
Several not-so-hard oxides are reasonable polishing agents and can remove and/or reform the soft
hydrated layer. In general, optimum polishing rate coincides approximately with Moh's hardness
for the polish of around 6.5, very close to the hardness of most glasses.
The best polishing agent - as regards rate of glass removal and ultimate surface finish - is
cerium oxide slurried in water. This oxide contains the potential polyvalent cerium atom and redox
reactions, due to Ce(IV)/Ce(III), may well be providing chemical assistance to the breaking up of
the silicate lattice. In addition mobility within the hydrated layer around the ceric ion also plays a
role. Certain substances can act as accelerators for the polishing process. The best appears to be
Ce(OH)4, i.e. CeO2.2H2O, precipitated fresh, in situ, in the polishing slurry from a soluble Ce(IV)
salt, that probably is involved in an equilibrium reaction.

SiO2 + Ce(OH)4 <--> CeO2 + Si(OH)4

In addition, transient formation of [..Ce-O-Si..] complexed groupings has been suggested.
Analytical techniques have identified Ce atoms incorporated below the final polished surface,
perhaps from such intermediates. The breaking, and reforming, of Si-O bonds is probably aided by
the transfer of OH groupings to incipient fracture sites by a transport mechanism using the
relatively large and mobile coordination sphere around the oxophilic cerium atom.
The cerium concentrate (predominantly cerium oxide), derived from bastnasite, is an
excellent polish base. In addition the oxide derived directly from the natural-ratio "rare-earth"
chloride, as long as the cerium oxide content is near, or above, 50 %, provides an adequate glass
polish; the polishing activity is better than the CeO2/LnO ratio would suggest. Materials, prepared
prior to any Ln purification steps, are the source for the lowest cost polishes available that are used
to treat TV face plates, mirrors and the like. For precision optical polishing the higher purity
materials, based on separated cerium oxides, are preferred.


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 Post subject: Re: working with honduran opal
PostPosted: Thu Sep 26, 2019 2:05 pm 
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Hi both !
Ducan, thanks for the explanation, there is no doubt that chemestry is the most beautiful art of nature !
Stephen, it appers to be more felsic indeed, but im not sure and yes, igneous rocks is a crazy world. As far as i understood, cerium oxide is soluble in sulphuric acid, but i could also use a magnifying loupe with a needle to clean the white spots, will find the right solution when i see the white spots.
The hardest part is to get the epoxy no way i can get opticon products :(... I think i'll use normal epoxy... You guys think that vacuum would work better for impregnation than the heating configuration?

One more question, can i round the edges with sand paper? Which grit?

Thank you both!


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 Post subject: Re: working with honduran opal
PostPosted: Sat Sep 28, 2019 4:47 am 
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TiagoPT wrote:
The hardest part is to get the epoxy no way i can get opticon products :(... I think i'll use normal epoxy... You guys think that vacuum would work better for impregnation than the heating configuration?


You can buy a bottle of opticon online easily. You can even find it for sale on ebay for US$30.


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 Post subject: Re: working with honduran opal
PostPosted: Sun Sep 29, 2019 10:36 am 
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Dioptase wrote:
TiagoPT wrote:
The hardest part is to get the epoxy no way i can get opticon products :(... I think i'll use normal epoxy... You guys think that vacuum would work better for impregnation than the heating configuration?


You can buy a bottle of opticon online easily. You can even find it for sale on ebay for US$30.



Hi,
I can't they don't ship to Portugal (trust me i've checked).

PS: I really like your name =D


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 Post subject: Re: working with honduran opal
PostPosted: Sat Oct 05, 2019 1:58 pm 
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Hello all,
just to update, today i started to do the sugar treatment, i will leave it in the hot sugar solution for 4 days (+/-) and then the funny part. will keep you guys updated :)


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