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.
Types of Silver Solder
Classified based on melting temperatures. There are basically three options:
- Easy – melting range 705 to 725°C
- Medium – melting range 720 to 765°C
- Hard – melting range 745 to 780°C
Cadmium Free
Cadmium was used in silver solder to reduce the melting temperature. Typically 16-25% cadmium. Cadmium melts at 320°C and when heated above this, which it would when silver soldering, forms cadmium oxide. Serious health effects on the lungs and kidneys are the major concern with cadmium oxide. Hence it was banned as an additive except for aerospace, defence or safety critical applications.
Quite frankly the latest cadmium-free silver solders are so good that there really isn’t a need to use cadmium containing ones.
Forms of Silver Solder
Silver solder comes in a few different forms:
- Hard metal rods – these are typically round or flat strips
- Flux coated hard rods
- Silver solder paste – a mix of solder and flux
Flux
Flux is defined as: a material used to hinder or prevent the formation of oxides and other undesirable substances in molten metal and on solid-metal surfaces, and to dissolve or otherwise facilitate the removal of such substances.
In any form of soldering you will need a flux. For silver soldering fluxes contain potassium salts or fluorides and borates. Normally these come in powdered form and are mixed with water.
Applying the flux with a brush to all surfaces prior to assembly works really well. This way you can ensure all surfaces are fully coated with flux.
There are also ways of masking areas to inhibit the spread of the silver solder.
Coupon Testing of Silver Solder
The flux with enable the silver solder to flow via capillary action through the joints. In this article we look at where the silver solder is applied and how robustly it spreads.
Brazing Torch
With brazing torches there are different sizes. Also, different types depending on the gas and whether air or oxygen is supplied in addition to fuel.
Micro Brazing Torch
A small torch that uses refillable butane and fits in the hand is a great benchtop heat source. These are capable of silver soldering small parts.
Hearth
When silver soldering with a brazing torch you will need some form of hearth. This will help protect your working area and reflect heat back into the workpiece.
Homemade Brazing Hearth
This simple to make hearth was made using bricks from a wood stove. These are low cost soft bricks that can be cut with a wood saw and fixed together with woodscrews.
How to Silver Solder with a Torch
Below are the steps required to get a good quality silver soldered joint.
- abrasively clean the areas of the parts to be soldered
- clean the parts in a pickling acid – cleanliness is key
- dry the parts
- apply a flux
- place the part in the hearth
- heat the part
Kiln Firing
A lot of commercial silver soldering is fired in a kiln. Once setup this gives high quality and consistent results.
Silver Soldering in a Prometheus Kiln
Silver soldering in a small home workshop kiln is a bit of a challenge. It took a few goes with different forms of silver solder and temperatures to get this to work reliably.
Anti-Flux for Masking off Areas
There are a number of options for masking areas where you don’t want the silver solder to go.
- Pencil Lead
- Tippex
- China White
- Yellow Ochre
- Jewellers Rouge
- Rubber
- Soapstone
- Anti-flux Commercial Products
A graphite pencil works rather well and I tested it as a silver solder masking tape for copper and brass sheet.
I will gradually expand these with actual examples so that you can clearly see and experience silver soldering.
References
I have included a number of references that will help expand your knowledge of silver soldering:
- “Exposure to cadmium in silver soldering or brazing“, HSE
- “Metal joining products and services“, technical pages from Johnson Matthey who are also one of the main produces of silver solder and fluxes.
Fixing the Burrell Nameplate
Silver soldering this Burrell nameplate was probably over the top in terms of requirements. However, this was actually quite easy to do with an easy flo paste.