While you can mill roasted malts such as roasted barley in your normal grain mill, to get the best colour and flavour use a good quality burr (not blade) coffee grinder to get an extremely fine grind, almost like dust. The Gaggia grinder (shown in picture) is the one we use and recommend when making beers such as our Dry Irish Stout. You'll use so little roasted malts in even the darkest of beers that it doesn't take much time to pass the small amount required through a grinder.
Grind the roasted malt fine (I use a setting of 10) and add it after the 90 minute mash rest is complete. This avoids lowering the mash pH too far and reduces the chance of astringency which can occur from over-steeping highly roasted grains. Once the 90 minute mash is over, stop the mash pump, add the roasted malt, and give it a good stir to mix it into the existing grain bed. You need to stir well as otherwise the fine layer of powdery roasted barley on top may stop the flow. Start the mash pump again and continue with your mashout. The wort will be cloudy again but it will clear as the grain bed rises to mashout temperature over 20-30 minutes.
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