This is my first cider. I threw in some grapes I'd frozen a while ago. I bottled directly from primary. 2 weeks in the bottle and I checked on it, saw the sludge on the left. A night in the fridge and then poured off the clear cider into one glass and the sludge into another. So much sludge. A good 15-20% of the volume. Lesson learned - rack to 2ndary for clarity sake.