This is the answer I posted on BF:
I will wade into try and explain the terms used. The wording can confuse people who don't know the difference. I work with solders and brazes every day.
A solder is a eutectic alloy of metals that have a melting point lower than the individual ingredients. It can be as simple as adding 10% tin to 90% lead ( plumbers solder) or as complex as mixing 90% platinum with 10% gold to make high temperature hard solders for welding platinum.
Soft solders are mixtures of low melting metals that will melt in the range from 200F to 700F. The metals used are commonly: Lead, tin, gallium, indium,silver,cadmium, and some others. Plumbers and electronics solder are not for knife work.
The flux used has to match the metal mix. For soft solders, these are zinc-chloride solutions and other similar liquids. The acid fluxes and paste fluxes used in plumbing work are not for knife soldering. Stainless steel needs a solder specifically made for that use. Not all soft solders will work on stainless steel. Stainless steel also takes a different flux.
Solders used where the joint will show to some degree are made "Brite" by the addition of small amounts of silver, usually 2-4%. These are referred to as Silver Bearing Solders, and are different from silver solders. Stay brite is one of these types. In truth, this really is just sales hype...it isn't much brighter or stronger.
Now, to complicate matters worse, we have the word HARD.
Hard solders are alloys of higher melting ranges, from 700F to over 2000F. Most are in the range from 1000F to 1350F.The silver content is what usually determines the hardness. It runs from 20% to 95%. The term "silver brazing" is usually applied to the lower silver content ( below 50%), and the term silver solder to those above 50%.
Hard solders are typically alloys of silver, copper, tin, cadmium, and some others ( jewelry solders are mixes of gold,platinum, and the alloy metals). These need different fluxes, and require higher heat, than soft solders. To make matters worse, they are divided into use classifications of soft, medium, and hard. Soft is now usually called "easy" to help with the nomenclature problems.
Marketing is everything in the sales world, and the folks who sell soft solders want ways of saying that their solder is better than the rest. So, they use phrases like, " The world's hardest soft solder", and " use the term " silver solder" and "silver bearing solder" as if they were the same thing.
A simple rule is that if you can cut the solder with a knife blade it is a soft solder, and if you need scissors, it is a hard solder.
The most common soft solders found in knife supply catalogs are the Stay-Brite, and Eutectic 157 types. Jewelers prefer TIX solder ( "The World's hardest soft solder"), as it is stronger,and comes in thin wires. A few knife suppliers carry it, but all jewelry suppliers have TIX.
So,
You can have a soft hard solder, or a hard soft solder....
You can have a 430F silver solder and a 1350F silver solder.....
You can use a heat gun or an acetyl/ox torch......
see where the confusion comes from!!!!
Now, for some soldering info:
All solder joints require smooth surfaces...and complete cleanliness.
No rough sanded surfaces, traces of dirt, or oils.
The surfaces must mate well. Solder isn't a filler compound, it is a jointing material. It flows by capillary action . The closer the joint, the better the flow. This is up to a point though. If the joint is clamped too tight, no solder can flow in at all. This will result in a solder starved joint....which will fail.
Soft solder isn't a structural joint. Hard solder is. If the joint is mainly being sealed,as in soldering on a properly fitted guard, then Stay Brite is a good choice. If you are soldering on a threaded tube of nut to a pommel cap, and it will be tightened , then you need a hard solder ( brazed) joint.
The joint will need to be chemically cleaned before the solder will flow. The proper soft solder flux is usually the one supplied with the solder. For hard solders, there are commercial fluxes, depending on the material being soldered, but Battern's or Griffith's are pretty universal.
Apply a small amount of flux to the joint, making sure it wicks into the joint fully. Warm this with indirect heat. After it heats up, apply more heat, and apply the solder to the junction of the joint surfaces. Don't expect the solder to jump up and run over to the joint...put it where it needs to be. Once the solder starts to melt, remove the heat, and only re-apply it if needed. The main cause of soldering failures is too much heat...not too little. Heat sources can vary a lot. For soft solders, a heat gun is excellent. Any small flame source will work - a small pencil type butane torch; a hobby mini torch (looks like a cigar lighter); a propane torch ( JTH-7, etc) ; or a small jewelers torch, like a Smith, Midgett, or Little Torch. For hard soldering you will need a torch with air/oxygen and gas mixed. The small jewelers torches mentioned are superb.
Keep the flame size and temperature as low as it can be and still get the job done. Hissing bright blue flames, and billowing yellow flames are both bad.
If the solder isn't flowing like you want, applying more heat won't solve the problem. Stop, clean everything again, and start over. If the flux burns when heating, start over again. If a joint is flowing, but seems to need a bit of encouragement, remove the heat and apply a small amount of additional flux. Re-apply the heat just enough to make the joint flow.
The use of a "solder Pick" is advisable. It is usually a piece of brass or steel wire/rod with a sharp point. You run this along the joint as the solder melts, and it breaks the surface oxidation and surface tension to make the solder flow perfectly. Also, if there is a small place where the solder didn't flow, running the pick back and forth over that spot will usually make it flow in.
When done with the soldering, clean the joint and surrounding area well. Wash in soap and water ( scrub well), rinse, neutralize any flux with windex or ammonia, rinse again. Dry well and apply a light machine oil to prevent any creeping brown fuzz in later months. The clean-up steps can't be over emphasized. This is the main cause of rusting and discolored joints.
