How to solder a copper joint with a gas torch

January 28, 2025

Soldering looks like a dark art from the outside and it really is not, because all you are doing is getting two clean bits of metal hot enough that molten solder gets pulled into the gap between them, and nearly everything that goes wrong with it comes back to one of two things. Either the joint was not clean, or it was not hot enough. Sort those and the solder does the rest on its own.

I am self taught on most of what I know about tools, so I will tell you what actually matters rather than what sounds impressive.

One thing to get straight before you start

Plumbing work on the water supply in a house is licensed work in South Australia, so this is not me suggesting you go and replumb the bathroom on a Saturday. What a torch and a bit of solder is genuinely for is the shed end of things, so an irrigation repair, a bit of brasswork, a low voltage electrical joint, sheet metal, jewellery, or just practising on offcuts until your joints come out looking the way you want them to.

What is actually in a torch kit

A basic kit is a brass torch head, which is the part that mixes gas with air and gives you a flame you can adjust, plus a gas cylinder that screws onto it and a hand sparker to light it with. Some heads have an igniter built in and some do not, and the separate sparker is worth keeping in the drawer either way, because a built in igniter is usually the first part to give up.

Which gas to put on it

Propane is the usual one and it will handle most jobs around a house or a shed. MAP gas runs hotter, which is why it turns up in plumbers' utes, and it is what you want when you are heating something with a bit of mass in it that keeps pulling the heat away as fast as you put it in. Butane is the mildest and it suits small precise work where you would rather not cook everything around the joint. They are not all interchangeable, so check the thread on your head before you buy a cylinder to suit it.

How to solder a joint

1. Clear the area and put your gear on

Move anything that will burn, and that includes the dry grass you are standing in if you have dragged the job outside. Gloves and eye protection go on before the gas does, and you want somewhere the fumes can get away from you rather than a shut up shed.

2. Clean both halves of the joint

This is the step people skip and it is the one that decides whether the joint holds. Take steel wool or a wire brush to the end of the pipe and to the inside of the fitting until the copper comes up bright, and then keep your fingers off the cleaned surface, because the grease off your hand is enough to stop solder flowing where you want it to go.

3. Flux it

Flux is what stops the metal oxidising the second it gets hot, and without it the solder just balls up and sits there looking at you. A thin even smear on both surfaces is plenty. Slathering it on does not make the joint any better and it leaves you more to clean off afterwards.

4. Put it together and check your gas connection

Assemble the joint so the two parts are seated properly, then screw the torch head onto the cylinder, firm rather than gorilla tight. Before you light anything, brush a bit of soapy water around the connection and watch for bubbles, which is the same test we do on every gas bottle we hook up at the shop and it takes about 10 seconds.

5. Light it and set the flame

Crack the gas open a little, spark it, then bring the flame up to where you want it. You are after a flame with a defined blue cone in the middle rather than a big lazy yellow one, because the tip of that inner cone is the hottest part of it and that is the bit you aim at the work.

6. Heat the fitting, never the solder

This is what separates a joint that holds from one that weeps six months later. Play the flame around the fitting so the heat goes in evenly, and keep the solder out of the flame altogether. When the metal is hot enough, touch the end of the solder to the far side of the joint and it will pull itself all the way around the gap on its own. If it balls up and rolls off instead, the metal is not hot enough yet, so take the solder away and give it more heat rather than aiming the flame at the solder.

7. Let it cool on its own

Do not throw water at it to hurry it along, because cooling a joint that fast can crack the thing you just made. Once it has gone properly cold, wipe the flux residue off with a damp rag and have a good look at the ring of solder around the joint. You want an unbroken bright line the whole way round, and a gap in that line is a leak waiting to happen, which is a lot easier to redo now than after it is buried in a wall.

What else a torch earns its keep doing

A gas torch is one of those things that sits in the cupboard until you own one and then you find jobs for it constantly, because it will shrink heat shrink onto a trailer wiring repair, put enough heat into a rusted bolt that it finally lets go, thaw a frozen tap or a frozen lock on a cold morning, and get a fire going in a wind that keeps blowing your matches out. There is even a propane weed burner built on the same idea, if you would rather burn the weeds off a path than spray them.

Nobody lays a perfect joint first go

The people who make this look effortless have a few hundred ordinary joints behind them, and that is the whole difference. Buy a metre of copper and a handful of fittings, solder a dozen joints you fully intend to throw away, then cut a couple of them in half to see whether the solder actually went all the way through. That is twenty minutes of your time and it will teach you more than any amount of reading, including this.

And if you are standing in front of the torches wondering which one suits what you are doing, just ask us in the shop. There are no stupid questions in a hardware shop, only expensive ones you find out about later.

Shop related products

Hot Devil Purified Butane, 5 pack. Refill gas for butane torches and gas soldering irons.
Promax WG16 Welding Gloves. Leather gloves that will take the heat when the fitting will not.
Ugly Fish Sparkie Safety Sunglasses. Certified eye protection you will actually keep on your face.

Did you know we also stock propane torch kits with a hand sparker, pencil flame torch kits, spare brass torch heads, Hot Devil propane and MAP gas cylinders, hand sparkers, acid core and rosin core solder, silver solder, Bakers soldering fluid and flux, and steel wool in every grade from coarse down to 0000. Most of that never makes it onto the website, so give us a ring on (08) 8536 4333 to check what is on the shelf.

The rest of the gear sits with the hand tools if you would rather have a look through it yourself.

More from the shed

I am the sort of person who still cuts a practice joint in half to see whether it went all the way through, and I am not the least bit sorry about it.

Cheers,
Candeece

Candeece