How to avoid cinnamon sludge in a beverage?
The cinnamon "sludge" is fiber. The fiber that came from the cinnamon is soluble in water.


10 grams of cinnamon is about 8 grams of fiber (see http://en.wikipedia.org/wiki/Cinnamon).
It is safe to eat / drink (fiber!) in moderate amount (Chinese cinnamon or cassia is not safe in high dose because of its toxic component called coumarin).
Of course, cinnamon sticks won't produce so much sludge.
If you use a hot drink, stir it so it doesn't sit at the bottom.


Ceylon cinnamon ("true" cinnamon) is different from Chinese cinnamon ("cassia") and may have different properties with regard to solubility and water. Check out your container that doesn't sludge, and compare with what you have been using in your grinder.