Flux for silver braze

[Swede]

I need to do some silver soldering today. It's been a while and I can't find my regular flux! I am using Harris Safety-silv for the job.

I have some Lincoln brazing flux that contains boric acid and borax. Will this be suitable? If not, is there any "home brew" substitute? I've got a lot of various chemicals left over from other projects.

 

[Ed T]

Sounds right. Test drive it on some scrap to see how it works.

 

[moanaman]

Silver solder flux and brazing flux are totally different, one is not a suitable substitute for the other.

 

[Springbok]

I agree with moanaman please go to the Cup Alloys web site they are UK based and give serious advice on this subject
Silver soldering is a minefield of controvesy in the UK and Europe at the moment with this metal haveing cadmium in it.
(silver solder) for your own safety I would not experiment with chemicals.

Have a safe and prosperious new year

Bob

 

[Ed T]

My response was based on:

1)Swede said he was planning to use harris safety-silv alloy which is a cadmium free, hard, silver brazing alloy with 30-60% silver depending on the particular flavor he has. So, I knew he was really talking about silver BRAZING, not, technically, silver soldering.

2) My favorite flux for these materials is "Handy Flux" which is made from boric acid and borax in a water based paste form which is the same mix he described for the Lincoln product.

The difficulty here, I think, is the often casual use of "silver soldering" when what is really meant is "silver brazing". I do it myself and there are many posts relating to silver soldering when silver brazing is really what is meant. So, silver soldering flux which is usually a solution of, I think, zinc chloride in water will certainly not work for silver brazing. It is used for "silver bearing solder" like Sta-Brite which goes on at around 500F.Plenty more than I want to write about the details of the differences between the two processes is available all over the web for those who are interested. Basically soldering is bonding with dissimilar filler material at temps below 850F, if memory serves , and brazing is anything hotter than that. IMHO Swede would not be at any risk in trying the Lincoln flux on a test piece so that he can move on with his project. His call.

 

[terryd]

Hi,

When I was at Teacher Training College in the 70s one of my courses was in Jewellery (UK spelling) and Silversmithing. As our work was to be hallmarked it was important to use high silver content Silver Solder. We simply used a borax paste for this work by using a borax cone in a ceramic dish. the dish was flat bottom, a drop of water added and the cone ground into a paste. To reuse when dry, simply add water into the dried dish and regrind the cone. It is a simple, cheap and effective flux at all temperatures and grade of silver solder. There are still plenty available on the internet, for example:

http://www.jewelsandtools.co.uk/products/Jewellers_Tools_and_Equipment/Soldering_or_Melting/Soldering_Fluxes/Borax_Cone_and_Dish.html

By the way, what we call silver soldering i.e. a high silver bearing filler rod at temperatures between 650 and 800 ºCelcius (1200 - 1470 ºF) is just that and not a low temperature process which we call soldering - soddering in the USA - probably from the French 'souder' for solder, (they also use the term braser to refer to all soldering, again derived from latin for brass). The term 'Braze' is a derivative of the Old English word for Brass and the (derivative Bronze is also based on this family of words as is the word Brazen meaning 'brassy'). So to be precise 'brazing' should only be used for the process of joining metals with a brass filler rod, but of course it has become synonymous with any high temperature joining process using a filler rod in many parts of the world. Hence it's use to describe silver soldering in some places and by some users - "Nations divided by a common language". All we need to do is to understand and respect these differences

 

[tel]

1)Swede said he was planning to use harris safety-silv alloy which is a cadmium free, hard, silver brazing alloy with 30-60% silver depending on the particular flavor he has. So, I knew he was really talking about silver BRAZING, not, technically, silver soldering.

Actually, technically, silver soldering Borax mixed to a paste with clean water will do fine.

 

[Swede]

I should have used correct terminology in the OP.

I try to use the term silver brazing for those processes that take place at 1200 f, dull red. I kind of wish that the old timers never called it silver soldering because I think it's a different process, and 9 times out of 10 if you ask for "silver solder" at a generic hardware store, you're going to get "silver-bearing solder", the stuff that flows at hundreds of degrees lower.

The stuff I used is definitely a braze. Anyway, the Lincoln flux worked fine, but my limiting factor when I tried it was the BTU needed to get the assembly up to temperature.

I did a bit more thorough search, and most of the silver braze fluxes have some fluorides and a few other exotic ingredients, but the borax/boric acid mix did OK as far as I could see.

Thanks for the suggestions. Now I just need to get more heat into the assembly to get it to flow properly.

 

[Stan]

Just to add to the confusion, when working with copper based alloys (brass and bronze) you can use Sil-Fos (2% to 15% silver and 1200F to 1400F) without any flux. The phosphorous in the rod acts as a flux.

Sil-Fos is only a fraction of the price of the high silver content silver solders and for many uses it is equal or better.

 

[GWRdriver]

Sil-Fos (2% to 15% silver and 1200F to 1400F) without any flux. The phosphorous in the rod acts as a flux. Sil-Fos is only a fraction of the price of the high silver content silver solders and for many uses it is equal or better.

It's good for cold water pipe and refrigeration line work, the purposes for which it was formulated. But phos-coppers are subject to enbrittlement at elevated temperatures and decompostion when exposed to sulphurous atmospheres (ie, coal smoke) and for those reasons is not recommended for model boiler work, and in fact is prohibited in the UK an Oz. Another problem is lack of flow. Many things we must join in model engineering, boilers bits in particular, requires the solder we use to flow. No flow, no penetration, no connection. We also regularly need for solder to run into a joint and be virtually invisible. It's all about flow and the constant discussions about cadmium bearing silver solder, and its future availability, bear this out. No doubt phos-coppers have their uses but it doesn't flow and that alone eliminates it from 98% of the work I do.

 

[Stan]

GWR: Agreed. It is not for everything, just like there is no universal brazing product. If you are making a buildtup brass flywheel with lots of brazing, it is a lot cheaper. The lack of flow makes it ideal for fillet joints.

 

[Springbok]

I must say it is extremely interesting the differences of terminology in engineering between the UK and the USA, and before joining this forum I had not realised how great the devide had become. A project for someone to create an engineers lexicon of the differences in terminology between us. A challenge but if this thread is anything to go by a hard task.

Everyone no matter where you are in the world have a pleasant, prosperous and all round good new year, and lots of fun in the workshop.

Bob

 

[Swede]

^^ Very true, the dreaded "silver steel" called for on British ME plans being one of the most common. That, and the EN alloys and the like, create confusion. Then the British see callouts for SAE-based stuff, and it's baffling. ;D