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Glassy Flux Residue

I tend to use a lot of flux. After soldering it will usually dissolve either in the pickle or rinse bath after pickling. Once in a while there is still a glassy flux residue left that is very hard and difficult to remove mechanically. I find that, if I leave the piece in water for a while it all dissolves. Once in a while I find a very small pit in the metal where the glassy flux was before it dissolved. The pit isn’t deep enough to be a problem, but I am just curious if anyone can tell me what happened to cause the pit.

I sometimes have had a problem when casting using commercial fluxes that may have a high boric acid content, It seems to stick to the fluid metal pour more than straight borax, and I’ll find it’s residue on the sprues and even on the casting itself displacing metal. I no longer use boric acid for fluxing purposes other than mixed with denatured alcohol as a fire scale preventer. I’m wondering if you may have a problem similar to what I’ve seen. Now when casting, and sometimes when soldering I use straight borax powder. For most soldering, I use a liquid flux like Batterns, and rarely a paste flux like Handy flux. Most pits in solder are caused by a contaminated joint and that would be the first place I’d look. As you don’t name the type of flux or brand used, it’s hard to give you a more definitive answer.

I use water diluted Handi flux most of the time, but the pits aren’t in the joint, that would make sense, the pits are in the metal some distance from the joint. This happens very rarely, but it is curious. Regarding casting; I only cast ingots. I will sometimes get red smears of what appears to be glassy flux from the borax that I use. A graphite stir rod seems to remove it if I remember to use it. Thanks…Rob


I use Boric acid and alcohol almost exclusively as fire coat and just a drop of generic green flux only at the joint. I rarely have an issue with pits in the solder or around the area I am soldering. But when it it happens to me it seems to happen more often with lower temperature solders that are being over heated. I like boric and alcohol because it is an easy fire coat and hot water will wash it away.

I just bought some TSP and I thought I’d try some Prips.

Brother Don

---- Robert Meixner wrote: