Salem

Lets assume that i have coal what have lead purity 50 and ore purity 50 on mercury, when make bar with those does it increase or decrease overall purity of iron bar? Or if i have high mercury purity ore did i need also use high mercury purity coal to get better end result.

coal does not matter in the slightest, 20% ore = 20% bar and so on

Four purities are long gone from Salem. Only one purity that matters is Aether.
Iron purity is average of leftovers in smelter and ore, so it is quite possible that putting same purity ore into smelter the results (if it happens to yield two bars) will be slightly different.

When dealing with ore of different purities it is best to have a number of smelters, each dedicated for certain purity range, so leftovers have less effect. Note that metal purity does not matter except for blades.