Black Ash Mortar

Q I have a project in Bolsover, Derbyshire which requires that we match a black ash lime mortar (the house is ex-coal board housing stock and a black ash slag would have been readily available when the house was built). I'm having trouble sourcing an aggregate which will give us the very dark grey colour that we need.  I'm sure some other BLF members will have been down this road before and wondered if you have any recommendations?

A Some BLF members have tried to replicate black ash mortar using ash from traditional sources. They have found this to be an unsuitable source of fine aggregate since the ash contained significant contamination with oily residues and excessive fine dirt. They have had success by using 'dark' fine aggregate from the Ludlow area with a graphite black pigment