Silver Soldering for DIY Brewing Vessel Upgrades

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If you can solder or "sweat" copper plumbing parts together reliably, you can use that skill to modify your stainless steel brewery too!

First, why would we want to do this?
While vendors are making excellent strides in engineering better and more reliable weldless components for different kettle/tank applications, some folks would still sleep better knowing they were not relying on gaskets to keep their liquid in the tank.
For years, the only alternative for most people was finding a local professional welding shop to TIG weld various bungs or nipples into the vessels. This solution is fraught with problems including high labor cost, sketchy work quality, and in some areas of the country plain inaccessibility. Most welders that are capable of doing sanitary or even passable quality welds on stainless steel are in such demand that you can't afford their time on a hobby budget. On the other hand, when someone suggests they are capable of doing the work inexpensively and actually has the time to do it, you are in for a display of how NOT to weld stainless. Sorry. This is a realistic situation and a thread titled "show us your welding nightmares" would fill up pretty quickly with pictures and tales of woe.

Soldering, in some situations, is a happy medium between weldless and welded fittings. Silver brazing and silver bearing soldering has been around forever, but it took a while to catch on for home brewing applications partially due to perceptions of strength issues and lack of appropriate fittings for the job.
SIDEBAR: Let's clear up some terminology. If you say "silver solder", old timers will assume you mean using filler of about 60% Silver and temps of over 700F similar to brazing. The topic of this article is for soft soldering using Silver Bearing Solder in the 4-6% Silver content range which only requires temps of 430F. It's probably best to just call it "soldering".
Required Tools and Materials:
  • Portable propane torch (very inexpensive option is the Bernzomatic UL100 Basic Propane Torch Kit and it even includes propane as mentioned below).
  • Disposable propane or MAPP gas tank
  • Stainless steel wire brush or sandpaper
  • De greasing spirits such as acetone, lacquer thinner, or denatured alcohol.
  • Solder: Several brands are out there but the basic requirement is that it is listed as safe for food contact and a Silver content of 3-7% is preferable.
Silver adds strength and resists thermal expansion cracking so more is better. However, you can't go much higher than 7% without it being more of brazing filler. The Harris Stay Brite or Staybrite #8 is easy to get and comes in a kit for smaller jobs.
Flux: Flux is a solution that etches away the oxide layer on your base metal and allow the solder to adhere to the raw metal. No matter how clean your surface looks, the oxide layer is either still there or will form when you start heating. Flux keeps this from repelling your solder. Again, many brands are available but they have to be specifically capable of working on stainless steel. Harris's Stay Clean LIQUID flux works very well but heed the bold specification of LIQUID flux as the paste version with the same name will NOT work.
Surface and Fitting Type Requirements:
Unlike welding we are not causing the two materials to fuse together but rather using solder as an adhesive like epoxy or wood glue. Furthermore, solder does not like to build up into a bead to bridge large gaps either. For this reason, the most successful joints have the fitting in a great amount of surface contact with the vessel wall. Long story short, you will not have much success drilling a plain hole in the side of your pot and sliding a coupling into that hole. Even if you drill that hole very precisely, the most contact you have is the inside edge of the hole in the pot (less than 1 square cm of total contact area).
To get a much stronger physical joint, the contact area should be increased by using a fitting that contacts the face of your vessel wall such as a welding spud as seen in the pictures. Welding spuds are functionally similar to an NPT half coupling, but a large flange is cast into them to account for the surface area we need. You can create this large contact patch with off-the-shelf part in a couple creative ways.
You may thread an NPT lock nut down over a pipe nipple and solder the lock nut to your vessel AND to the nipple to keep it leak free. On joints that will take a LOT of abuse, you might sandwich your vessel wall between two lock nuts providing increased mechanical support. At that point, the solder is really just providing the liquid seal.
Another increasingly popular method of achieving more contact patch is to undersize the hole in your vessel wall and use a makeshift tool and die to pull some of the vessel wall outward. This process has been called dimpling, pulling, swaging or circular flaring. The most difficult part of doing this is finding materials to build the mandrel and die. Most people use butt weld reducing couplings for the mandrel and a larger straight coupling for the backing die. There are several discussions right here at Homebrewtalk regarding different parts to use that range from cheap (and scary) to all stainless and expensive. User Nostalgia also has a great video showing soldering with the use of the dimpling technique:

Basic Soldering Technique:
Once you decide exactly what you want to attach to your vessel, the basic process of soldering is summarized here:
Drill the appropriate sized hole and then scuff up the contact areas of both the vessel and the flange using some 100" grit sandpaper then wipe off residue with solvent on a rag. Apply acid flux such as Harris Stay Clean Liquid flux to both surfaces. Using an LP or MAPP torch, warm the vessel walls first keeping the flame moving, avoiding the fitting for a few minutes. Then concentrate the heat on the fitting itself, avoiding burning the flux as much as possible. The ideal temperature is reached when your solder melts when applied to the joint between the flange and vessel and NOT when the solder wire is directly heated by your flame. Solder should be quite liquid and start drawing into the gap. Work the solder around the entire fitting, reheating the fitting briefly as necessary but always remove the solder before applying heat because we are not trying to melt the solder with the flame. This is very important basic soldering technique so it bears repeating. We use the flame to heat the work and the work melts the solder.
If you find that the solder is melting and sticking to only one of the surfaces; the vessel wall or the fitting, it is likely due to insufficient flux application or not enough heat on the surface staying "dry". You can try squirting a little flux on the work, but don't over do it or you'll cool everything down too much. If it is a heat issue, try applying more heat to the surface that is not wicking in the solder.
If you overheat the work, especially when there isn't enough flux in the joint area, you may build up enough oxides on the surface that reapplying flux is not going to work anymore. In this case, it's best to clean everything up with a stainless steel wire brush and start over. To avoid overheating, keep testing if the solder will melt when applied to the work. You only want to heat the work to the point where solder will be liquid and no hotter. You can also use an IR thermometer to test the surface temp and you are looking for a 450F. If the temp drops very quickly, the surrounding area is still too cold and acting as a heatsink.
Disclaimer: Soldering is not horribly challenging, but you should be moderately handy if you're going to attempt this. If you've never soldered copper plumbing with success before, you may want to do some practicing first even on copper tubing and other scraps. Acid flux is dangerous to handle and the fumes are toxic. Please use a respirator and/or solder outside or in a well ventilated area. Torches make things very hot! DUH... typical silver bearing solders with 3-6% silver content melt just under 500F and the flange will stay hot for several minutes after you remove the heat.
Some more soldering videos:
Prep Work:
Soldering a welding spud:
Soldering a surface mount triclover flange:
Is it strong enough?
Epic HBT Thread: Soldering Stainless Steel
Please keep in mind that I'm not attempting to take credit for coming up with this method of brewery modification or even suggest that I'm better at the techniques than anyone else. This article is meant as an overview and reference to help you decide if it's something you'd like to try on your own.
Parting fact: This soldering technique is also very handy for joining stainless steel to brass or copper.
Nice article Bobby. I managed to do about 12 couplings on my set up with no issues after this method started being discussed on HBT. After years of use, they still look like I just finished them.
Good info. I did a solder on my Bk and it worked pretty good. I used the draw technique to pull the smaller opening outward and create a stronger joint.
For the heating element I soldered a large nut to the side of my BK where the hole was, and can thread the element directly to the nut, with a waterproof electrical box to contain the wiring.
The problem with that is that the kettle is not flat and I did not get good fill where the wall bends away from the nut.
it functions, but it drips a tiny bit from a pinhole. My next attempt I will flatten the keg where the heating element goes in and it will make the solder operation a lot easier and definitely more leakproof.
Great article Bobby!
1" welding spud from BrewHardware would have been a good solution for you. http://www.brewhardware.com/fittings/120-spuds. Anyone looking into this note that it won't work for dip tubes and other fitting meant to thread from the inside of the kettle. They have external NPT threads.
Great article Bobby!!
Is there any method you offer that will allow this to be done with ball valves and sight glasses?
The 1/4" NPT weld spud works great for sight glasses. For a ball valve, it's a less obvious choice because you typically want a diptube on the interior. I touched on it in the article but I think the diptube assembly that has the long threads on it would solder in quite nicely while using a locknut on the exterior for strength. Essentially you'd install just like it would be weldless and then you leave out the gasket and solder everything, including the threaded joints to make it permanent (ish). Duh, that's another thing I forgot to mention. Soldering is less permanent and can be changed out if necessary. Not so for welding.
I used silver solder on the 1" element nut two years ago. It worked perfectly, is liquid tight and it's and extremely strong bond. No problems to this day.
Clear and well written article. Nice work!
It gets to the point on purpose and method to achieve excellent results.
Thanks for the summary, and the videos.
I just soldered two 1/2" NPT welding spuds onto my keggle, for a valve and thermo. Got tired of leaky weldless connections. The silver soldering worked perfectly and cleaned up nicely. My first time soldering stainless. Thanks again for a very helpful article!
Pic at:
Easier than I thought it would be. A while back I was able to successfully solder spuds, NPT couplings, and TC flanges to a couple of kegs I converted to electric. I followed Bobby's advice found on his site and in his videos and everything went to plan. No problems since. Thanks Bobby!
Oh, one tip I can offer: if you're soldering a new connection adjacent to an existing one, you'll need to wrap a wet rag around the existing parts so you don't unintentionally de-solder them while heating up the new parts.
Thank you Bobby for the video. Its so well done it looks very professional. I been looking for ways to add a fill port for my nano boiler and your video clearly took me thru the whole step. I have one question how hard to do the same thing on the side wall of a 4" kitchen SS canister, instead of the bottom (the canister is 11.5" long). I am a bit concern the surface is concave (or convex depending on where u look from) will be problem mating to the flat surface of the fittings and its a bit flex when applying pressure to the sides. Any thoughts?
Thank you