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Lapidary / Gemstone Community Forum
December 11, 2018, 06:43:23 pm
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What to use to back a cab?

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Author Topic: What to use to back a cab?  (Read 2024 times)
Haderly
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« Reply #15 on: November 04, 2014, 03:53:00 pm »

330 epoxy is the standard for doublets and dries water clear. I recently seen a new black 220 epoxy. The old 220 epoxy dried a amber color but the new black stuff is something I want to try on a future project.
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PhilNM
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« Reply #16 on: November 04, 2014, 04:04:34 pm »

330 2 part clear epoxy.
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rocks2dust
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« Reply #17 on: November 04, 2014, 04:58:33 pm »

Black polymer clay! I can't believe this hasn't occured to me from the start. I can mold it right inside the setting and bezel to compensate for the stone height and bake it right in, at which point it should become inert. Does anyone know if that would be safe for the silver and the stone?
Polymer clays won't work over the long term. Those do contain PVC and plasticizers which are bad news for metal  (the acids they release over time are very bad for silver, copper and some stones—I've personally experienced severe etching damage and silver disease after only a few years stored in proximity to PVC). On the other hand, hi-fired PMC-type metal clay would have all the binder burn out, leaving just metal, but I don't know whether anyone makes a black bronze clay.

If the back of the cab is uneven, you could set it in black plaster (lamp black + plaster) or hard pitch. Both were used for backing gems in Georgian and Victorian mourning jewelry (even some of King Tut's gems were set using bitumen that had been boiled to get rid of the volatiles so that it wouldn't soften or ooze at normal temps). Some petroleum tars are high-sulfur, though, which might cause problems on silver and copper. Similar can also be made from conifer pitch + charcoal. I haven't heard of significant corrosion problems with old gems set in pitch, though it could also be that damage did occur in many piece, which we don't know about simply because they got trashed.
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finegemdesigns
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« Reply #18 on: November 04, 2014, 11:54:14 pm »

I just did these tonight with Epoxy 330 and Black Jade Actinolite:



Almaden Cinnabar from Spain. Slab is 1.3mm thick. I will cut these out tomorrow after epoxy sets.
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GregHiller
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« Reply #19 on: November 05, 2014, 06:20:39 am »

>I have been using black electricians tape for backing/padding on crystaline Opal with good luck.<


I've also been using electrical tape.  Highly inert.  If the stone is opaque I tape it directly to the back to fill in any gaps and make the setting more solid.  It's easy to trim with a scissors once it's stuck on.  If the stone is clear I tape it to the silver sheet in the back of the stone.  I don't tape it directly to the stone if the stone is clear as it's possible the adhesive might not stick evenly.  I generally use the very thick form of the electrical tape (you have to look a little harder for it at Home Depot).

For the opposite effect (white or silver backing, works great for Montana agate!) I now use the inside of my salad croutons bags. 
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PhilNM
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« Reply #20 on: November 05, 2014, 11:32:03 am »

Another one I forgot is Jet. Makes a nice black backing.

I'm running out of basanite, anyone know where to buy some nodules about 4 to 6 inch size up to ?

Thanks
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mdfa.ca
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« Reply #21 on: November 05, 2014, 02:13:41 pm »

Thank you, Rocks2dust for the info on Polymer Clay. I thought it was supposed to be inert once cured/baked, I guess not...  It would seem the more I get into this the more complicated it becomes. I truly don't want to attempt a doublet because if I screw anything up, well, it's a nice stone and it's not mine. So I really just don't even want to take those chances. I think I have made up my mind to try the J B Weld product. I'm not going to mate it to the back of the cab, just make a little ramp for it inside the setting, and maybe, if it's not black enough, paint it with model paint?

However, if I wanted to try make a doublet, on another stone, where would I buy the black jade / basanite / jet? And which one works the best?

GregHillier,

When I read your mention of electricians tape I was also surprised. That stuff is so gunky! I can't imagine that it would not start releasing the adherent, especially after being repeatedly heated through contact with skin. Which brand do you use?
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rocks2dust
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« Reply #22 on: November 05, 2014, 03:55:03 pm »

Jet is easier to work, but weaker, so best used to back thicker, sturdier stones. Basanite and jade are very tough, and good for backing thin, fragile slabs (opal, turquoise, etc.).

Another one I forgot is Jet. Makes a nice black backing.

I'm running out of basanite, anyone know where to buy some nodules about 4 to 6 inch size up to ?
I have a few cobbles left that you can have for the cost of shipping (and I'd be glad if you'd send mdfa a small slab, since my saw is put up and I no longer have slabs). PM me with your postal code, if interested, so I can weigh and tell you what the PO will charge.
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PhilNM
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« Reply #23 on: November 05, 2014, 04:01:55 pm »

PM sent.... if mdfa can wait, be glad to, as long as he/she too pays postage, I think he/she's in Canada by his/her nickname?
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PhilNM
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« Reply #24 on: November 05, 2014, 04:09:53 pm »

I truly don't want to attempt a doublet because if I screw anything up, well, it's a nice stone and it's not mine.

If you're putting a backing of any kind on any stone, you're making a doublet. Even just backing it with JB weld, it just became a doublet, but with one side a gemstone, and the other the backing... so go ahead, you have nothing to lose.
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mdfa.ca
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« Reply #25 on: November 05, 2014, 06:58:04 pm »

Rocks2Dust and PhilNM,

Thank you both for your generosity. I'd love to have a small slab to practice on and I'll gladly pay the shipping price. And I do know what you are saying, Phil, but I won't actually be attaching the cab to the JB Weld. I'm going to form it separately and just sandwich them together in the setting. Well, if it works.

Oh, and by the way, I'm female LOL. I thought I introduced myself a way back but it may have been a different forum.
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PhilNM
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« Reply #26 on: November 05, 2014, 08:27:54 pm »

Rocks2Dust and PhilNM,

Thank you both for your generosity. I'd love to have a small slab to practice on and I'll gladly pay the shipping price. And I do know what you are saying, Phil, but I won't actually be attaching the cab to the JB Weld. I'm going to form it separately and just sandwich them together in the setting. Well, if it works.

be sure to use saran wrap between the Jb and everything so you can easily peel it off after you mold everything and it cures.
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mdfa.ca
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« Reply #27 on: November 06, 2014, 08:15:56 am »

Will do! :-)
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Haderly
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« Reply #28 on: November 06, 2014, 08:48:13 am »

I think adhering something to the back is a better solution. The slight air space will give a duller look when looking through the cab. I have doublets that I bought in a estate sale that were glued with superglue and have started to separate. The separation from the backing is very apparent and looks bad compared to the area that is still attached.
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PhilNM
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« Reply #29 on: November 06, 2014, 10:42:58 am »

True. That's why it's always better to glue it on them do your grinding and polishing.m But if she's adamant that she wants it separate, than all we can do is tell her the best way to do things. Personally, I like basanite on stones like turquoise, flower agate, etc, but JB weld on the less expensive material. Basanite can get expensive in the long run. You waste more rock that the resulting slab, so rock that costs $4 per pound ends up $16 per pound usable.... plus cost of epoxy, etc etc.... But it sure ends up pretty! There's a local equivalent stone that's a strong that I can use, but it's brown, not black. Doesn't look near as nice. Historically tho, the local NA silversmiths used to use it for backing before they discovered LP vinyl.
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