Using Highly Roasted Grains in the Mash
A few words of paraphrased wisdom from Mary Anne Gruber of Briess Malting Company which may assist your quest for great beers.
The highly roasted grains, Black Malt, Chocolate, Black Barley, Roasted Barley, are used for their coloring and flavor contributions. They do yield some extract but the yield is quite low. They give up the color and flavor quite readily.
With this in mind, it is not necessary to add them at mash in but somewhere down the line. The amount of flavor needed is dependent on the beer style. This, and your equipment, will determine where the roasted grain is introduced.
For a rich chocolaty, coffee flavored Porter, Chocolate and Roasted Barley can be added to the kettle part way through or at the end of boil. The spent grain will come down with the trub in the whirlpool.
For a strong Stout, add Black Malt and Roasted Barley toward the end of mash or in the lauter. Add the dark grains to the top of the lauter bed to avoid a slow run off.
Other Specialty malts (i.e. Aromatic, Biscuit, Victory, Melanoidin, etc.) should be mashed for the greatest benefits and to avoid soluble starch carryover.
Refer to January/February, 2002 Zymurgy for information on Cold Water
Extraction of Dark Grains.