Silver Solder Coupon Tests

A few silver solder coupon tests to look at options for soldering copper and phosphor bronze (PB). The test parts are a representation of the PB saddle and copper boiler on the miniature traction engine.

Preparing Test Parts

silver solder test parts

Fundamentally a sheet of copper 1.2mm thick has a PB M4 bolt through the top and into a threaded PB plate.

The copper sheet is approximately 40mm square and the PB plates approximately 20mm.

All parts were pickled for 20 minutes before being dried and flux being applied.

For consistency in this article all images on the left will be for the case where the silver solder is laid next to the PB plate. On the right is the case where the silver solder was applied in a loop around the bolt.

silver solder test flux applied

The flux was applied wet using a brush. The powdered flux was mixed with a small amount of water.

Flux was applied to all of the surfaces that were to meet. Including the bolt and the copper plate interface.

solder test parts
The two test conditions. LHS: silver solder applied in two strips either side of the bronze plate. RHS: silver solder applied as a loop around the bolt head.

Silver Soldering

silver solder test open flame heat

Both parts were heated in an open flame. Heat was maintained on the part for around 30s after the solder melted.

No further flux or silver solder was applied during the heating process.

silver solder test post soldering

Initial Results

silver solder test parts
Note: I had to flip these images to get the orientation correct, so they are a mirror image.

Looking from the bolt head side (ok there is not actual hexagon or slot, this is just a round stub). The left hand side has a much neater bead of silver solder around the bolt head. On the RHS where the silver solder was applied as a ring the result is much more messy.

The interesting part is that on the LHS the silver solder has been drawn through the thread of the tightened joint.

silver solder test parts
Again, flipped images. Just allows me to keep left and right hand side in correct order.

On the PB plate side of the sandwich you can see that on the LHS part there is a much thicker bead of silver solder around the plate as expected. However, although the silver solder had tracked through to the bolt head it is not visible around the thread of the LHS part.

On the RHS part the silver solder has tracked through from the bolt head surround. It can be seen around the bolt and around the perimeter of the PB plate.


Both parts were sectioned: hacksawed, milled and then ground on a few grades of wet and dry. These were then scanned side by side. The sections show that the silver solder has more completely tracked all of the joint interfaces on the RHS part.

In the LHS image you can see a number of voids in the bolted joint.

section silver solder test
Scanned sections of silver solder coupon tests

Zoomed in the voids are very clear.

Silver solder coupon tests
Zoomed in with the LHS part at the top and RHS part at the bottom.

These silver solder coupon tests are a bit crude. However, both parts show that the silver solder tracks through the joint interfaces. Also, carefully applying a wet flux to all surfaces that form the joints is a good idea.

A few more coupon tests would be useful to look at repeatability and other options such as silver solder paste. These will also build confidence before silver soldering the boiler.

brazing hearth

Brazing Hearth

A very simple to make brazing hearth that has lasted more than 4 years and is used for brazing and heat treating.

brazing hearth

Silver Soldering

Silver soldering is one of the most common metal joining techniques in model engineering. Also, used extensively by jewelers. Hence there are a number of references. This page is based heavily on model engineering experience and applications.

Leave a comment