DIY instructions:

"You will need:
1. concentrated nitric acid (HNO3)
2. mercury (Hg) - from chemist or from thermometers
(alternative - buy mercury nitrate)
3. sodium or potassium thiocyanide (NaSCN or KSCN)

First, dissolve mercury in nitric acid to get mercury nitrate solution.

With concentrated nitric acid your mercury nitrate will probably crystallize - add sufficient clean water to dissolve the crystals (say, 10 times the volume of nitric acid used). (Alternatively: dissolve crystals of mercury nitrate in water)

In separate container, dissolve some sodium/potassium thiocyanide in water. Add thiocyanide solution to mercury nitrate solution and mix it well. Greyish suspension of insoluble mercury thiocyanide will form - wash with water a couple of times (add water, mix, wait for mercury thiocyanide to separate at the bottom, decand/remove clear water, repeat.

Put paper coffee filter into funnel and filter out the mercury thiocyanide, dry it - must be bone dry to use it.

Safety:
Nitric acid is dangerous, handle with safety glasses, wash your hands immediately if spilled. Vapours of mercury are dangerous - best gandle on open air. Sodium or potassium thiocyanide is not that dangerous but don't leave solution for anyone to drink!

When dissolving the mercury in nitric acid, vapours of nitrogen oxides will form - these are poisonous if inhaled.
While decomposing, mercury thiocyanide forms vapours - don't know what these contain but don't inhale!!!"