For general purpose tongs start with a piece of 3/8" x 1" flat bar about two feet long or a little longer.
For small light duty tongs start with 1/4" x 1" flat bar.
Difficult to obtain 5/16" by
Mild steel (or even wrought iron) is satisfactory for tongs.
Do not use carbon steel over 40 points carbon.
If you intend to make goose neck or offset tongs start further up the bar leaving extra material to shape after the tongs are assembled.
You can always take material off but its harder to put back on.
The lifting tongs above are modified goose jaw bolt tongs that the jaws had been accidently burned off when I found them (scrap tongs).
The remaining jaws were reshaped and then 1" x 1/8" (25 mm x 3.2 mm ) flat bar was arc welded on and shaped to fit the crucible.
Crucible tongs need to be carefully fitted to the crucible so that there is a wide smooth area of contact.
Crucible lifting tongs are only used for removing the crucible from the furnace.
A pouring shank or pouring tongs are used to fill the molds.
Crucible lifting tongs should fit about 1/2 to 1/3 of the way down from the top of the crucible.
On standard crucibles the tongs should fit just below the "bout" or largest part of the curved sides.
On A-line crucibles such as the one above about 1/3 down from the top is correct.
Note that crucibles lose strength at working temperature so fits should be smooth.
Crucibles also expand from the heat and tongs will fit lower on a hot crucible than on a cold one.
This is not noticeable on small crucibles but can be significant on large ones.
The pouring bowl "tweezer" tongs are made from 1" x 1/8" x 32" (25 mm x 3.2 mm x 810 mm) mild steel flat bar and were cold bent in a vise and over the anvil horn.
The resulting tongs are 15" (380 mm) long and fit both small crucibles above.
For small crucibles these are much more convenient than other types of tongs.
Using a light weight refractory lift body furnace these tongs can be used for the entire casting process.
Many jewelers and other doing small castings melt metal directly in the pouring bowl using a torch and also do not need lifting tongs.
Note that early and primitive smiths used wooden tweezer type tongs with a leather hinge to handle hot metal for forging.
While these do not have the leverage of modern tongs they do get the job done.
It is something to keep in mind if you need tongs and nothing else is available.