Just a couple more pics.
The first pic shows two completed knobs, with the original knob used for the casting mould in the centre. After fitting the brass ferrule, each knob was drilled and tapped 4BA and a 4BA brass grub screw fitted. The knobs were then sprayed with acrylic white primer, and acrylic gloss which was a good match for the original knob as can be seen.
The second pic shows a knob in the mould making jig from a Roberts R200, which has sloping sides, but as stated earlier, despite that, the alginate is sufficiently flexible to enable the knob to be wriggled free from the alginate without damaging the mould. (Leastways, I hope so!)
One thing I've learned is that when lowering the knob into the alginate, 'air-locked' bubbles can form under the lower part of the knob. (That's the front of the knob in its normal use). To prevent that, alginate can be smeared onto the underside of the knob with a finger prior to lowering the knob, to ensure that air isn't trapped when the knob is lowered into the alginate.
Although the wood-filler is quite satisfactory in terms of the finish and durability, I'm inclined to the view that a more runny casting medium will do a better job. I'll try the Super Steel epoxy as and when I get time.
I'm nuts, of course - I don't actually need any knobs just now, and all of this is ludicrously time consuming, but it's all good clean fun, it stops my wife or kids putting me in a home, and it's in my skills set, except I'm now getting high on the fumes from the wood-filler!
Needless to say, I'm not in a position to be able to offer anyone my services as a knob caster!
Hope the thread has been of interest. I've got many other projects at various stages of 'un-completion' , but just went off on a frolic to have a go at this on impulse. Quite why, I don't know, but then I'm not a doctor, so how could I?.