By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale May 23

Silver soldering problems.

All Topics | Latest Posts

Search for:  in Thread Title in  
Brian Abbott13/01/2019 00:10:57
381 forum posts
62 photos

I am hoping someone will be able to offer some advice.

I am currently making the small linkages for my Minnie traction engine.

The problem I am having is when I silver solder them, instead of the solder flowing round and creating a nice smooth fillet its creating what is in the photo.

If this was constant then I would blame the solder but its not, some have soldered fine, others not.

I am using the old type solder, easy flo No 2 I think it is, and have made sure the joints are clean.

what i am doing is instead of feeding the solder in once everything is hot i am placing a small sliver of solder next to the joint and using the heat to pull the solder through, but i have done this on each joint and as i said some are ok.

For scale the boss diameter is 3/16”

Any thoughts ?


Jeff Dayman13/01/2019 00:33:12
1500 forum posts
37 photos

Hi Brian, I'd try a bit more heat and also add a bit more flux just before the solder melts.

The method of leaving a bit of solder next to the joint, well fluxed, is a good one and I have used it many times. I have noticed on small workpieces though, like your linkage, a lot of the flux boils off during initial heat. If you add a bit more flux mid-heat it usually gets the ball rolling and the joint flashes solder in right away. Again you may need a wee bit more heat also, judging by the crystallizing markings on the solder.

Good luck.

John Alexander Stewart13/01/2019 04:59:26
747 forum posts
51 photos

Q: Do you tightly clamp the 2 pieces?

Q: Do you heat from the "other" side as the silver solder is at?

Q: Do you leave a small gap for the solder to go into?

Thought 1: You might want to centre punch some marks on one of the pieces to give that hairline gap for the solder to flow through. I centre punch *everything* that I'm silver soldering now.

Thought 2: As Jeff says, more heat and more flux. I sometimes use a "poker" rod to add more flux. Inside, I use a plumbers propane torch, which is really not good for much more than silver soldering really small stuff. You do want the pieces to come up to temperature, especially the inside of the joint....

Keep going, practice does help.

Danny M2Z13/01/2019 05:22:56
731 forum posts
278 photos

Lots of useful info here **LINK**

Although I just stick to the soft lead pencil dont know

* Danny M *

JasonB13/01/2019 07:04:04
15337 forum posts
1580 photos

The "textured" surface to your fillet rather than a smooth one is more likely to be overheating which makes the solder gas.

Does not look to be any problem with it wetting the surfaces or flowing.


Edited By JasonB on 13/01/2019 07:05:04

Brian Abbott13/01/2019 22:34:16
381 forum posts
62 photos

Thanks all for the reply.

Spent a happy hour tonight trying a few things, the problem is I think that as the part is so small it is cooling to quick and the solder is not having time to flow.

I initially did a few samples to try different heat / solder / flux combinations and all came out fine, the one major difference is that the plate I soldered the boss to was wider, maybe about 3/8 not the original 3/16”

On this occasion I can make the parts bigger and cut the plate down, but am guessing better torch control to keep heat in the part whilst the solder solidifies is needed, or solder it on a big lump of copper.

Brian Abbott14/01/2019 22:42:12
381 forum posts
62 photos

Hello, sorry back again,

Still seem to be having problems, for small jobs like this can i ask what torches people use,

My current torch came with a 7.7kw burner which is what i use for all my soldering,

Thanks in advance.

Edited By Brian Abbott on 14/01/2019 22:48:01

CuP Alloys 115/01/2019 10:01:07
186 forum posts

Hello Brian

It looks like Jason has a point. From the picture it looks like the alloy has been overheated. The "gas" coming off the alloy (if it is easiflo - and you don't know?) is likely to be cadmium fume.



I suspect that you are preferentially heating the alloy. Steel does not conduct the heat quickly into the joint area. The flux doesn't work and the alloy doesn't penetrate. So you then heat the alloy to get it to flow. Ring any bells?

Heat the joint. Not the alloy.

Get the joint hot. Watch the flux to see what joint temperatue you have. Let the alloy get its heat from the workpiece.

Fit a smaller burner than the Sievert 2901 that you are using. Be patient while heating. Make sure that the joint is up to temperature.

Change to a cadmium free silver solder.

Burners, alloys and technical information in an excellent book is available from CuP Alloys on line or at Ally Pally this week.

Take care.


PS You don't get information like this from ebay, car boots or many other sellers of silver solder.

What has your supplier had to say? Let me guess!!!!!!!!!!

CuP Alloys 115/01/2019 19:38:13
186 forum posts

Ooops !

Typo in the post.

Should read "Sievert 2941burner"


Brian Abbott15/01/2019 22:40:15
381 forum posts
62 photos

Thanks Keith,

Which dia solder and which burner do you think would best suit these small parts ?

Once the London show is out the way i will get them ordered.


JasonB16/01/2019 07:07:03
15337 forum posts
1580 photos

Brian, how are you positioning the work when heating?

If you have it laid flat on your insulating material much like in your photo then the round part will tend to get hot quickly and the flat strip take a while to come upto temperature as some heat will be wicked away into what it is laying on meaning you end up overheating the end and boiling the solder until the rest comes upto temp.

Try it with the flat part raised off the surface, I use a couple of short lengths of 1/2" steel angle stood outside cornet up, this then has a very small contact area so heat is not easily transfered. you will then find the flat part heats up quicker. Another option is to leave it on the full length of 1/4 x 1/16 flat bar and support that well away from the area to be heated.

Myself I only tend to use 1.5mm rods and feed in when hot enough (flux changed to liquid) the only time I may use short pieces placed on the job is when using up short ends. Don't usually change to a smaller burner just don't wind the knob open so much and position the flame depending on size of part.

Brian Abbott16/01/2019 12:51:58
381 forum posts
62 photos

Thanks Jason for the reply.

I have the part sat on a small piece of Vermiculite while I solder it, but must admit that now I am looking a lot closer the boss is getting hot well in advance of the plate and the solder is defiantly fizzing away which I guess is overheating.

I tried again last night, this time with a standard type small blow torch, spent longer heating the plate and not the boss and fed the solder in once everything looked ok, results where good but I think I was running out of gas so heat was a problem.

Will try elevating the part and see what happens.

Thanks again.

Ps..will order some cadmium free solder..

CuP Alloys 117/01/2019 09:06:10
186 forum posts
Hello Brian.
Suggest that you use 455 alloy say 1.0mm dia.

Direct the heat more into the thicker, heavier component.

For details on alternative burners see the sievert range on

You may also consider using a longer life flux ie HT5.

The cause of your difficulties is very common - you have veered away from the basic principle of brazing of using capillary flow to effect your joints.

For more information and avoid any future difficulties see "The Guide to Brazing and Soldering" available via the website.

Paul M20/01/2019 13:06:16
18 forum posts


For silver soldering to be successful you must make sure all parts are clean. The solder has to flow through the joint by capillary action and will not if the parts fit too tightly, flux has not been properly applied and importantly all parts are not brought to a temperature to melt the solder. Where possible direct heat more towards the thicker material and gradually heat thinner material. This requires a torch that is suitable for the size of job. Too much heat will break down the effectiveness of the flux, metal can oxidise. I am not a great fan of adding solder to the joint prior to heating where material is of different thickness and size, I prefer to heat the material and apply the solder after heating it and dipping in flux. Over time you get used to the correct red heat and get used to the flux changing as you reach the correct heat. This is why I tend to solder without too much direct light on the hearth. For most joints you shouldn't have to chase the solder around the joint if you have got the work to the correct heat.

Do you use a hearth with fire bricks (or whatever you wish to call them)? It is best to have firebricks behind your work to reflect heat back onto the work especially if using high melting point solder.

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Latest Forum Posts
Support Our Partners
TRANSWAVE Converters
Eccentric Engineering
Eccentric July 5 2018
Allendale Electronics
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest