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Dry calendaring or hard nip calendaring can be done with two or more rolls and one or more nips. This type of calendaring is detailed in a separate training module. Wet stack calendaring is typically done with three or more rolls and two or more nips. In a wet stack, water is added to the sheet surface one or more nips using water boxes. Uniform nip pressure profiles are critical to prevent water pockets from forming, which can cause sheet breaks. The added moisture and applied nips pressure develop good smoothness of the sheet. Dyes, lubricants, and binders like starch can be added to the water boxes if desired. After the wet stack, the sheet must be dried before it can be wound onto the reel.