I have used fire-bricks (Broken pieces from a coal fire back) without any problems for making a local hearth around the part. These sit in a larger hearth which is simply a few old bricks from electric storage heaters sat on the bench. (They have been there 30 years or so!). 2 make the base and 4 make walls around the outside. Great for small jobs with a single 2kW blowlamp. DO NOT USE HOUSE BRICKS _ THEY CAN EXPLODE WITH HEAT!
In fact I have another 10 storage-heater bricks that I use on a "temporary" bench for larger jobs. (model boilers mostly), where I have the space to use up to 4 blowlamps to develop a decent heat for the silver soldering of larger heavier items. I also have some large tins for filling with sand around 3/4 of a boiler for insulation. That also holds the job very securely. pieces of firebrick are used to localise the heat around the working part of the job.
I use 2 or 3 Paraffin blowlamps for "warming" the job, and a second-hand Sievert Blow-torch for larger jobs - from a 7Kg. Propane cylinder with regulator to 20psi. I also have a decent sized Swedish petrol blowlamp, as storing petrol safely is easy in my motorcycle... and Petrol is relatively cheap for fuel. (Paraffin ~£2/litre, Petrol £1.20/litre, Propane £3/litre! Disposable Canisters of Butane ~£7/litre).
I know the storage heater bricks "soak" a lot of heat, but the dense refractory seems to get hot on the surface and does not appear to penetrate far. For my boiler work, I like to cover the whole job in the hearth after doing some silver soldering, to allow the heat to soak and de-stress the job with slow cooling. Otherwise the huge contraction forces can break the job just joined.
the most important things I would stress:
- Ensure you have a safe work-place - free from other flammable materials, etc.
- Have a fire extinguisher to hand. I heard of a guy that "did a quickey" without his regular safety checks in place, dropped a red-hot lump of metal from the tongs onto his foot, and while he attended to his burnt ankle (much hopping and cursing!), the metal ignited the wooden floor - and it cost the whole garage/workshop!
- Check ventilation (soldering flux fumes are toxic!) and free access/egress from the hot-work zone.
- WEAR WELDERS' Leather protection. - I have an apron, gauntlets, and even a sleeve if I think I may need it. A job in a hearth gives off a lot of infra-red heat as well as a lot of gas exhaust heat. - 8kw into the job means you have the equivalent of 3 domestic room heaters within arm's reach while you are working. You will get hot! Even use aluminium foil to reflect heat from fingers of gauntlets, etc. - the backs of you fingers holding the flux applicator, solder and blow-lamp will over-heat and stop you from finishing the job properly otherwise.
- Wear safety glasses! Occasionally a speck of dirt or something can "pop" and send very hot particles into you eye otherwise. Can be VERY painful.. I know someone who lost an eye that way once.
- You need more heat than you think. If you get the job hot, but not quite hot enough, you have to stop and start again. Very frustrating. So use the biggest and best heating arrangement, as you can turn down the heat input, but can't turn it up more when working. Butane can be frustrating as the pressure drops noticeably with gas container temperature below 10 degrees C. It may be OK at the start of a job, then when you try to get the "red" heat, it just fails to get hot enough for silver solder to flow, because the canister has cooled itself! - So I use that gas blow-lamp to light the petrol blow-lamp for a proper job. Flame is slightly bigger, hotter and much more reliable. The large Propane blow-lamp is for use when I need to get the cylinder out of storage for a multi-lamp job. (Quick to turn ON or OFF and adjust when in use, while the paraffin lamps do the pre-heating.).
- Ensure you have a sensible secure set-up. Nothing worse than finishing a job to find it has slipped in joining, so you have a crooked crank or something! Often a vertical set-up will point gravity in a direction that holds the job right, without sag when red-hot and everything is loose. I use spare lumps of cast iron as "steadies", to wedge smaller lighter jobs to keep them from moving while soldering.
- Oh, and "Cleanliness, cleanliness, cleanliness" in the joint. (Plenty of flux - it washes off afterwards).
- Correct selection of fits (0.002~0.004" clearance for silver solder, NOT a close fit!), selection of solder grade and flux for the job and materials.
Conclusion: Done properly, this is a very satisfying process, and one I enjoy. Hope you do too!
K2