I was lucky enough to find some old cogs and clock parts from DH's Grandfather who used to mend clocks. I love the detail in them however realised making my own moulds would not be as straightforward as other items.
I used gel flex (blue) or you can used vinamould (red) to make the moulds, and silicone cupcake cases to pour the gel flex into. As the cogs have a very long spindle I needed to cropodile a hole in the middle of the silicone cases to make a suitable mould.
Once you have covered the cog you need to let the mould set and take the cog out. Then you need to cover the hole and pour more gel flex over the mould to finish it off.
I am still trying to make a suitable mould for the cogs with much finer detail, if anyone has any tips they would be greatly appreciated.