[stickle]

Everyone,

My in-laws lost their home in the recent Camp Fire in Northern California.  In picking through the wreckage we discovered two related problems that we could use your knowledge to solve.

They had silver coins in a "fireproof" safe.  The lining of the safe was black plastic.  The plastic melted and encased the coins.  Is there a solvent that will remove the plastic?

They had some other coins stored in ziplock bags.  These melted together with the plastic from the coin tubes.  Is there a solvent that will remove this plastic?

Is there a completely different way to go?  The black plastic gets soft at around 300 degrees F. but just smears. 

Your wisdom would mean a great deal to a couple of elderly fire refugees. 

Thank you.

S

[wildfyr]

I would try a good soak in paint thinner, aka toluene first.

If that doesn't work, then next I would try dichloromethane which is bought as paint Stripper in some places.

Be careful with both and DO NOT work with in enclosed spaces.

[Enthalpy]

Dissolving a polymer thicker than a paint layer tends to take very long. Nice if it works, but often discouraging. Ultrasound, like in a jewellery cleaner, often helps.

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An alternative might be an acetylene+oxygen torch. The flame burns the plastic away quickly, more so with an excess of oxygen. It may melt the coins too, depending on how it's done, but the silver in the coins has often the same value as the coins.

Molten silver is just more difficult to sell than a silver coin. Such a coin is basically a stamping meant to guarantee a silver amount and purity.

If you try the torch:
- Blow pure oxygen once the plastic burns. The flame will be less hot than with acetylene.
- Do that outside! Nearly all burning plastics are toxic for real.
- Maybe air suffices.

==========

Possibly a strong ultraviolet source would degrade the plastic at interesting pace, so it can be broken and brushed away, but I doubt it. Sunlight takes its time, especially if the plastic is black, thick, and resides between the coins. Some sources, like arc lamps, are stronger.

==========

As most chemical methods get slower with the plastic thickness, it wouldn't be bad to first separate the coins with a chisel, hammer (and vice), leaving just thin plastic on them.

After that, just a barbecue fire followed by brushing may suffice. De-oxidising the tarnished silver is standard practice.

==========

Hey chemists, would there be some compound (A bleach on steroids? Ozone? Percarbonate?) that oxidises the plastic away? That should be faster than a dissolution.

Or one that makes the plastic fragile (again an oxidizer?) so it can be hammered and brushed away?

Would a strong base be worth trying? Breaking the polymer chemically must be faster than dissolving it.

[wildfyr]

Pirahna solution? Highly dangerous but would fit the bill.

We don't know what polymer it is, and without FTIR or some manufacturer info we probably never will. If its polyester then KOH would be great. If its polypropylene then its useless.

[Corribus]

It's all blind speculation without knowing better what kind of polymer it is.  Polypropylene and polyethylene have much different stability to, e.g., UV light. I have no idea what a safe liner would be made from but plastic bags are almost certainly polyethylene, which is unfortunately very difficult to dissolve in anything, at least not without destroying the coin underneath as well. There's a caterpillar that can eat polyethyelene: https://www.nature.com/articles/d41586-017-00593-y

Honestly I think enthalpy's suggestion of controlled burning is probably the best bet. Silver's melting point is about 960 deg. C and manycommon organic polymers decompose at around 400-600 deg. C. If you could control the furnace temperature to be safely below the silver melting point but above the polymer decomposition temperature, you may be able to burn away the polymer while leaving the coins intact. Then clean whatever reside remains with a strong solvent. I wouldn't expect any kind of mint condition coin left over at the end though.

This does seem to defeat the purpose of a fireproof safe.

[Borek]

A lot depends on how fast you want to get rid of plastic. I agree with Corribus that polyethylene is easier burnt than dissolved, at the same time many years ago I have learned something interesting. I made a serious mistake - I left a polyethylene bottle near our fireplace. It melted, leaving a nasty stain on the grout. I have removed mechanically as much as I could, and a year later there were no traces left - apparently elevated temperature (in a place which can be typically safely touched with a bare hand) was enough to slowly decompose the polyethyelene. It can be impractical to keep something at - say - 120 deg F for several months, but it can work.

[stickle]

Thanks everyone for thinking about his problem and giving me some directions to explore.

Tried to post pics...not familiar with the process. 

So far...

problem-A large mass of coins (40 lbs.) encased in a monolith of melted plastic.  I used dry ice for an hour to make the plastic brittle then bashed it with a 2X4 and broke the monolith up, (is there a technical name for this process?).  Most coins still stuck together in units of 2s to 10. 

I wanted to start with simple and safe and proceed from there. 

Coins melted with clear plastic...
Soaked the worst burnt coins in white vinegar for a week.  Nothing.
Soaked others in Acetone.  Nothing.
Currently soaking another batch in a stripping product that contains toluene, I see some progress but very slow.

Black plastic coins-  In addition to what was done above, I put some coins that were stuck together in an "air fryer" (convection oven kinda thing) that has a temp control.  at around 300 F. I could get them apart, but the plastic is still on the face and sides of the coin itself. 

So I have time if time is the key variable. 

Maybe if I combine the processes...flame/acetone/heat something will happen faster...

I will keep you posted!

[wildfyr]

If you can continuously stir the stripping product bath that can speed things up quite a lot. Diffusion is slow.

[Corribus]

300 °F is only 150 °C. This is too low to see polymer decomposition on a reasonable timescale. You may have more luck with like a 500 °F oven. My outdoor ceramic grill with a pizza stone will get to about 750-800 °F if I really blast it with charcoal. If you don't have something like this, you might look around for someone with a kiln for firing clays and see if you can talk them into letting you try to bake your coins at low (for them) temperature. I'd shoot for something like 350 °C (650-675 °F) as a first try. This is where most organics will start to carbonize in a reasonable amount of time. Certainly, I would only try one coin at first to make sure you don't damage the coin, and make sure it cools before you start to clean the residue off because even if you're below the silver melt, it could be soft enough to damage by abrasion or bending. If you do it right, any carbon residue should come right off.

*Actually if you have any chunks of the plastic without the coin embedded, that makes a great 'spot test' material to see if you can get it to decompose.

[Enthalpy]

An electric oven with pyrolysis cleaning? Just, don't use the oven for cooking any more after that. Throw it away. And please don't sniff the gases coming out of the oven. Use the oven where an internal fire makes no damage.

Though personally, I'd go the flame method. Hold each coin in the flame with coal tongs. After that, brush the dirt away as possible and clean chemically.

Cleaning apparatus exist for jewelry with ultrasound. It could speed up some chemical actions, including dissolution.

[Enthalpy]

To hold the coins in a hot location, a kitchen sieve of stainless steel outperforms coal tongs. Mind any plastic at the handle.

A heat gun, often used to strip paint, would provide air at a controlled temperature.
https://en.wikipedia.org/wiki/Heat_gun
However, the plastic on the coins will ignite. You might build a small oven with a few loose bricks and blow the hot air in it.

My preferred version presently: melt and dissolve the plastic in a hot liquid. Among transparent liquids, cooking oil (maize, peanut) is a good choice, mineral oil is more a fire hazard, molten paraffin may dissolve some plastics better and is more transparent. Just do it in a pan (no reuse for food!) as for cooking, rely on the odour and fumes to estimate the temperature. The usual oil cooking temperature should be adequate for most plastics.

[P]

Thanks everyone for thinking about his problem and giving me some directions to explore.

Tried to post pics...not familiar with the process. 

So far...

problem-A large mass of coins (40 lbs.) encased in a monolith of melted plastic.  I used dry ice for an hour to make the plastic brittle then bashed it with a 2X4 and broke the monolith up, (is there a technical name for this process?).  Most coins still stuck together in units of 2s to 10. 

I wanted to start with simple and safe and proceed from there. 

Coins melted with clear plastic...
Soaked the worst burnt coins in white vinegar for a week.  Nothing.
Soaked others in Acetone.  Nothing.
Currently soaking another batch in a stripping product that contains toluene, I see some progress but very slow.

Black plastic coins-  In addition to what was done above, I put some coins that were stuck together in an "air fryer" (convection oven kinda thing) that has a temp control.  at around 300 F. I could get them apart, but the plastic is still on the face and sides of the coin itself. 

So I have time if time is the key variable. 

Maybe if I combine the processes...flame/acetone/heat something will happen faster...

I will keep you posted!

Do you want to sell the mess?  I collect coins and I would find it fun to remove them from the mess. I'll give you the spot price for the silver plus postage plus a small commission. I'll enjoy removing them and cataloguing them and keeping some and selling some on.  It is one of my hobbies. :-)   PM me if you want to sell them to me. :-) 

Regards,

P.

If the toluene is working, albeit slowly, keep it in there.  Maybe score up the polymer a bit before soaking to increase surface area.  Leave it for a few weeks and it will soften at least and you could then maybe hack the polymer off easier by a mechanical method.  Better yet - send them to me!  :-)

[P]

Do you want to sell the mess? 

That's a 'No' then is it?

 I used dry ice for an hour to make the plastic brittle then bashed it with a 2X4 and broke the monolith up

You risk scratching the coins - which won't really be a problem unless they are mint/UNC/proofs or in very good condition.

I used dry ice for an hour to make the plastic brittle then bashed it with a 2X4 and broke the monolith up, (is there a technical name for this process?).

Smashing. 

I do wonder though  -  if you are going to sell the coins afterwards then you might want to just sell the block.

[P]

Also - I don't think fire is a good idea.  You can get unwanted coloured toning on the coins when they take fire/heat damage.