There are three main techniques that can be called “granulation”: hard soldering, fusing/diffusion bonding, and colloidal soldering. The materials used for granulation are usually high karat gold and/or silver alloys as alloys below 18 kt. gold and sterling silver are not well suited to granulation (not that they can not be done by soldering).
So, What is the difference in Diffusion bonding and Colloidal or Eutectic Soldering?
When heated to well below the melting point, the atoms of the metal are vibrating, and as it gets hotter, they vibrate more. When two metals that have similar properties are close together, the atoms will mingle and form a bond. That is what I am referring to when I am saying diffusion bonding or fusing. When I refer to Fusing, I am generally referring to welding two metals of the same alloy together through the use of heat alone.
Refer to electron microscope analysis photos below of fusing Argentium/ Argentium, 22k Gold/Argentium, 18k Gold/Argentium. No solder or added copper to get these results.
Argentium Sheet fused to Argentium Sheet
AS Wire on sheet
When I taught Balinese metalsmiths how to alloy different colors and karats of gold, we made solders in each alloy.All work was then was done by sprinkling powdered solder over the entire piece. This technique is hard soldering.
18k sheet on Argentium
Colloidal or eutectic soldering is a technique that makes use of a colloidal mixture of organic glue and copper salts such as cupric carbonate or copper coating granules. This compound lowers the melting temperature of the two metals in contact (the granules and the base) after which the copper diffuses into both at the point of contact when fired in a reducing atmosphere. The glue burns away releasing the copper salt from the compound. The copper diffuses into the granules and the base linking them together.
22k on Argentium
When fusing Argentium to itself or other metals, I am using only heat and flux. The flux protects the surface from forming too much germanium oxide at high temperatures. It is a pure diffusion bonding and simple to do. I have had success in fusing Argentium with Brass, Copper, 22k and 18k Gold, Steel, and Platinum.