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.
cooking.stackexchange.com/users/16830/dnozay
After having to avoid consuming powdered cinnamon for months, I've recently found out the method to prevent it from turning into a slime when mixed with water or tea (hot or cold).
The trick is to mix the cinnamon with honey first .
You can mix 1/2 teaspoon of cinnamon with a teaspoon or two of honey, and stir it up very well so that the mixture becomes homogeneous as much as possible.
Then slowly add the water or tea while stirring the mixture.
A cup of mixture prepared this way will make a perfect drink.
Since honey is very healthy by itself, it can only boost positive effects the cinnamon has.
cooking.stackexchange.com/users/12587/Nikola Malešević
Use a two bottle system.
Make your cinnamon tea and let is stand overnight to settle, then decant into the second bottle leaving the sludge behind.
cooking.stackexchange.com/users/70177/George Beard
If I am going to drink hot cinnamon drinks i use a shaker bottle to keep the powder mixed into the drink.
cooking.stackexchange.com/users/71851/Art Scott
I use a french press to make my loose leaf tea.
All the cinnamon slime gets wrapped around the fine mesh strainer, and does not go into my tea or coffee.
cooking.stackexchange.com/users/73694/Anne Wall
I've tried all the already mentioned methods to avoid the "sludge factor" and all have been unsuccessful for me.
My solution?
I don't put the honey/cinnamon mixture into a liquid.
I make a paste of 2 teaspoons cinnamon stirred into 1 tablespoon of honey.
I use it as one would jam on toast or hot biscuits, or on apple or pear slices.
Make it ahead if you like as it will keep indefinitely refrigerated or not.
The mixture ratio was found online as one being most beneficial when taken daily to get the health benefits.
cooking.stackexchange.com/users/54611/Peacekeeper
It may help to use a mesh bag or strainer.
This doesn't precisely prevent the sludge, but it can help one avoid it in their drink.
mesh bags, or even tea bags, are available and often used for such purposes - allowing flavor to infuse out while keeping the less-desirable parts (like the fiber in cinnamon) in the bag, easy to pull out and dispose of.
It may be wise to consider pre-preparing your cinnamon infusion or tisane, especially if making a complex or thickly textured drink - this will let you get the cinnamon flavor without problems like heating ingredients not meant to be heated in a cold drink, or letting flavor infuse more easily into a thin liquid rather than a thick one.
cooking.stackexchange.com/users/47365/Megha
When you grind up cinnamon (say in a coffee grinder) and use it in a drink, it ends up producing this thick sludge.  You can only prevent the sludge from forming if you leave it very course.  Why does it turn to sludge?  I swear I once had a container of cinnamon that did not produce sludge.  Am I buying the wrong cinnamon?
How to avoid cinnamon sludge in a beverage?
cooking.stackexchange.com
<consistency><cinnamon>
cooking.stackexchange.com/questions/32730
cooking.stackexchange.com/users/3334/bobobobo
Ceylon cinnamon ("true" cinnamon) is different from Chinese cinnamon ("cassia"), and may have different levels of solubility in water. However, the 'sludge' is fiber that will dissolve easier if mixed with honey first or can be strained out with a mesh bag or strainer. Try mixing the cinnamon with honey first. Using a bag or strainer might also help.
The sludge is fiber. Try mixing the cinnamon with honey first and using a bag or strainer. Ceylon cinnamon and Chinese cinnamon have different properties with regard to being soluble.