Anti-soldering flux?

I just broke a drawer pull on a family heirloom desk. The pull consists of a cast brass base with a tab sticking out. The tab
is bent down through the ring, holding it flexibly captive. The tab is about 1/8" wide and half that thick, the ring about 2" diameter and 1/8" thick at the tab.
The tab broke where it was bent, so the easiest way to line up the break is to put the ring back where it belongs and solder the broken tab back in place. If the tab is soldered with the ring out, then some bending will be needed to put the ring back. That seems a good way to break the tab again.
If I can find something to coat the ring pull, so the solder won't wet it, I think there's a decent chance of success. I don't think lead-tin solder will be strong enough and I can't think of a reliable anti-wetting coating that will stand up to silver soldering temperatures. Aluminum foil comes to mind, and might work for soft solder but I don't think it'd take the heat needed for silver soldering.
Thanks for reading, and any ideas.
bob prohaska
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On Sat, 14 Oct 2017 01:46:24 -0000 (UTC), bob prohaska wrote:

30 years ago, soldering gold with gold solder, I would wet an artist's brush with chloroform and pick up the rouge off a stick with an artist's brush - then paint the mix on the work. Apply our flux, then heat with torch or oven to soldering temp.
Placement of the antiflux was very controllable. If the antiflux got somewhere we didn't want it - it wiped off smooth surfaces easily. After soldering it was kinda baked on, and would require a bit more effort.
I think you could use any fast drying liquid that does not leave any residue that might react with your flux. etc.
Might, or might not, work for what you're doing. Definitely run some trials before you attempt the real thing.
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On Fri, 13 Oct 2017 22:50:36 -0400, Bill wrote:

Rouge == red jewelers rouge, the stuff that's used on buffing wheels to polish metal.
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Test with "liquid paper" - it is used to keep damascus from sticking to the shell in forging.
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On Fri, 13 Oct 2017 23:09:11 -0400, snipped-for-privacy@snyder.on.ca wrote:

I have used that when silver brazing - works for me.
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"Bill" wrote in message
On Sat, 14 Oct 2017 01:46:24 -0000 (UTC), bob prohaska wrote:

How about... White Out? Its used in making canister damscus steel. That's a lot higher temperature than silver solder. Not sure how you would go about removing it afterwards though. Another option might be graphite spray lubricant. I use it all the time in lead casting, although the base metal (aluminum) of the mold does not wet easily to lead anyway.
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It seems as if correction fluid might be a pretty good bet. Near as I can tell the pigment is titanium dioxide, which is a fairly inert ceramic. It can be applied thick, perhaps it'll resist flux. Shouldn't be too hard to set up a rehearsal with some scrap brass for a test.
Speaking of setup, holding the parts in position looks rather difficult; the broken tab is only about 1/8" square. Can anybody suggest some sort of putty that will resist brazing heat, hold the (very small) parts in place and not sinter to something indestructible when heated? Maybe glazier's putty, but that can get rather hard. Fire stop caulking? modeling clay? Perhaps something made from household chemicals like talc and water?
Thanks for everyone's ideas!
bob prohaska
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"bob prohaska" wrote in message

It seems as if correction fluid might be a pretty good bet. Near as I can tell the pigment is titanium dioxide, which is a fairly inert ceramic. It can be applied thick, perhaps it'll resist flux. Shouldn't be too hard to set up a rehearsal with some scrap brass for a test.
Speaking of setup, holding the parts in position looks rather difficult; the broken tab is only about 1/8" square. Can anybody suggest some sort of putty that will resist brazing heat, hold the (very small) parts in place and not sinter to something indestructible when heated? Maybe glazier's putty, but that can get rather hard. Fire stop caulking? modeling clay? Perhaps something made from household chemicals like talc and water?
*****************
Modeling clay maybe?
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"bob prohaska" wrote in message

It seems as if correction fluid might be a pretty good bet. Near as I can tell the pigment is titanium dioxide, which is a fairly inert ceramic. It can be applied thick, perhaps it'll resist flux. Shouldn't be too hard to set up a rehearsal with some scrap brass for a test.
Speaking of setup, holding the parts in position looks rather difficult; the broken tab is only about 1/8" square. Can anybody suggest some sort of putty that will resist brazing heat, hold the (very small) parts in place and not sinter to something indestructible when heated? Maybe glazier's putty, but that can get rather hard. Fire stop caulking? modeling clay? Perhaps something made from household chemicals like talc and water?
*****************
Modeling clay maybe?
***
I used to use fire stop caulk all the time for construction when penetrating fire rated walls of communications cable and conduit sleeves. The stuff is really quite tough when cured. It might be quite hard to remove. I never did a post mortem on any that had been through a fire though.
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On Saturday, October 14, 2017 at 6:36:57 PM UTC-7, bob prohaska wrote: [about anti-flux]

Modeling clay (the greasy kind) or putty are no good. Casting sand and a little molasses is traditional for high heat, though. Or a mineral clay might work, I know painting a little clay slip onto copper wire keeps hot glass from sticking.
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On Sun, 15 Oct 2017 01:36:55 -0000 (UTC), bob prohaska wrote:

We used a soldering investment - something like this: https://whipmix.com/products/soldering-investments/
Soldering investment can be used in very creative ways to make an "index" that would hold your parts in a predermined relationship while the actual brazing is done.
I'm a bit concerned about the braze (whatever you use) flowing well into your joint. You might need to be very slective of the solder for it's flow properties, and/or carefully prepare (open up) the jointa tiny amount so your filler metal will penetrate the joint fully.
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On Sun, 15 Oct 2017 01:36:13 -0400, Bill wrote:

Here's a link to video of what was taught in dental schools 30 years ago that you might be able to modify/adapt to fit your needs. It shows it all. They're using hard, high strength, gold solder/braze. The brazing temp is not far below the melting point of the gold being used.
Duralay, the material that was used to tempiorarily splint the bits together is just a two part (liquid/powder) cold cure resin.
If you can find a suitable yellow colored, or gold solder, it might eliminate most of the gray/silver discoloration that regular silver solder can leave.
https://www.youtube.com/watch?v=7gcaGK5Zpnc

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Soldering investment looks like exactly what's required. Whether I have the manual dexterity to use it successfully is another matter entirely. The part I want to re-attach is slightly smaller than the dental fillings shown in the video, but the pull base and ring are quite a bit larger. That's not a good combination.
The video was absolutely fascinating. I can now understand why dental work is so expensive. Thank you very much for posting it.
bob prohaska
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On Sun, 15 Oct 2017 01:36:55 -0000 (UTC), bob prohaska

Do you have a jeweler's torch? You're going to need one. I wrecked a drawer pull about that size with my smallest torch.
Given my prior experience, I'd have a long look for a suitable replacement, just in case. There are lots of cabinet hardware outfits still around. I got my sister some "reed & ribbon" pattern stuff made in the USA. It was not cheap, as you'd expect for plated solid brass, but it was available.
Pete Keillor
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I do not; I have a very tiny butane-air torch and a small acetylene-air torch. The butane-air torch is probably too small, the acetylene-air torch a trifle big. I think it's got the better chance, however.

Fortunately, there's nobody breathing down my neck on this project. I broke the pull and will be very pleased if I can fix it. If I can't fix it neatly, I'll fix it less than neatly.....8-)
Thanks for writing!
bob prohaska
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On Sun, 15 Oct 2017 01:36:55 +0000, bob prohaska wrote:

Does the tab, when not broken, have enough of a curve that a bit of brass shim stock could be cut and shaped to lie on the outside of the curve and hold the parts in place; solder all three together.
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That would certainly help prevent re-breaking the tab. However, I then have one more part to locate correctly. Not sure I'm up for that.....
Thanks!
bob prohaska
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I tried "welding putty" among other terms in web searches and mostly got references to epoxy-metal concoctions that seem rather ersatz. I'll check with the local welding supply shops (Airgas and Trico in Woodland) but I think dental soldering investment comes very close to being what I need.
Thanks for writing!
bob prohaska
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On Friday, October 13, 2017 at 6:46:26 PM UTC-7, bob prohaska wrote:

There's a commercial product, Tix Anti-Flux, that should work.
Me, I just stay sloppy, and buff the excess off afterward.
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That in concept is exactly what I'm looking for. Trouble is, they say it's best with SO250 solder, which appears to be low melting point and has no strength or other specification. Since the original brass broke, I think silver braze will be necessary but I'll email them and learn what I can.

Can't do that in my predicament; if the solder flows between ring pull and plate there's no way to get it out.
Thanks very much for posting!
bob prohaska
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