Well the short answer… the one that works.
There are a lot of theories out there about what is best. And each metal smith that you ask will probably give you a slightly different answer. Much of it is based on what we are comfortable using and there are generally pluses and minuses with each type.
First though, safety. When using a torch, you need to protect the area that you are working on with a large, heat resistant surface. You can use sheet metal, or ceramic or slate tiles to create a heat resistant surface. Your soldering block goes on this surface.
When choosing your surface, you can choose colder surfaces to slow down the process, or a hotter surface to speed up the heating. If you are using a smaller torch, then using a hotter surface will help you be successful in soldering.
Hotter surfaces include: charcoal, fire brick, honey comb, solderite
Colder surfaces include: ceramic solder board, transite, silquar
Charcoal blocks create a reducing atmosphere (meaning it removes oxygen which creates oxidation on the metal) and reflect heat back on the article being soldered. It is inexpensive, soft enough to insert pins and can be shaped. It can also crack and will wear down over time. It can be messy and will need to be ground down occasionally on concrete or with coarse sandpaper – make sure you wear a mask when doing this. Wrapping a charcoal brick with steel wire will help hold it together over time.
Fire bricks, or kiln bricks come in different forms. They are lightweight, inexpensive and it is easy to set pins and shape the blocks when needed. They can be powdery so make sure you wear a mask when cleaning the surface on concrete or coarse sandpaper. Some kiln bricks are very porous and solder and other small bits can fall into the holes. I personally prefer the Vermiculite blocks as you can rub them smooth to eliminate pieces falling into the brick. One downfall of fire bricks is flux tends to harden on the surface and can be difficult to remove by sanding.
Solderite is much like fire bricks, except smoother. It also has a fast cool down time. It was made as a synthetic substitute for charcoal. It reflects heat back better than most solder boards. However, it can be burned and pitted by the torch, which means the boards can wear out faster than other soldering boards.
Solderite is much like fire bricks, except smoother. It also has a fast cool down time. It was made as a synthetic substitute for charcoal. It reflects heat back better than most solder boards. However, it can be burned and pitted by the torch, which means the boards can wear out faster than other soldering board.
Ceramic, Silquar, Transite Soldering Boards withstand the intense hear of a large torch but they dissipate heat quickly. They provide an easily cleaned surface for soldering, preparing your solder and more.
Lump pumice and an annealing pan is one additional item to know about. Volcanic pumice reflects hear really well and it is nice to be able to rotate your work as you solder or anneal. But don’t use this for small pieces that can get lost in the pebbles. This is great for annealing larger pieces of work, using the pebbles to support odd shaped pieces and the rotating pan allows for quickly changing positions.
So you might ask, what do I use? Well, charcoal is my main go to surface these days. Part of this is because I work mainly in silver and mostly in Argentium Silver. Argentium seems to like charcoal the best. Before that I used mainly a pressed vermiculite block, and still do use this block. Under my soldering block I have a Solderite board and that board sits on top of slate tiles. I also use an annealing pan and often put my charcoal or vermiculite block on the pan so I can easily move my piece while heating.
Hope this helps a bit… happy soldering.