Don't worry about grinding it. Soak your shredded bark in a small amount of acid solution (dilute vinegar, HCl etc) for a couple hours then freeze overnight. Thaw the bark ice block then add about 750mL of dilute per 100g of bark. Allow the resulting mixture (about 1L per 100g by now) to simme for 4 hours or so. You can then filter the bark out, and base the acid solution (or skip the filtering and base the mixture with bark left in). From here, extract as usual (nps pulls, a mini a/b cleanup if you want, and finally freeze precip/evap).
A good long acid soak with at least one freeze and one boil step should thoroughly lyse the cells and liberate all the alkaloids as water soluble salts.
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