punnisher

Gentleman
Would someone be kind enough to help sort my black powder problems?
1) I am not getting a fast burn
2) it leaves behind a pale blueish/yellow residue.

I am using Chempack sulphur chips from a garden centre, which look like pale grey hemispheres, ground down into a fine powder.
Lump wood charcoal of unknown origin again milled into a fine powder and then milled with the sulphur.
I tried to get Krista K but failed and came home with a sack of water soluble fertilliser with the following label
Potassium nitrate 13-04-46
Low in chlorine
Total Nitrogen 13.7%
of which Nitric Nitrogen 13.7%
Potassium Oxide soluble in water (k20) 46%

This came from a farmers supplier and so one would hope fairly pure etc but I have no training in chemistry and therefore have no idea if this is ok for black powder. It did say in bloody big letters on the sack 'Potassium Nitrate' though!

I mixed the chemicals by weight in the ratio 75:15:10 and then milled them for a few hours tipped some out and lit it

It would be great if you could help me to make really good black powder
Thanks