The Brew Bag is a tool from the Witchery mod. This tool is used to store throwable brews from within the Witchery mod. It cannot store drinkable potions, or Vanilla potions. It has 8 inventory slots, therefore it can hold up to 512 brews at a time. Shapelessly crafting this item with a dye will dye it to the according color.
Sneak and use the bag to open its inventory. Using the bag normally will open a radial GUI, where the brew can be selected. The brew's placement in the inventory determines its location in the circle. The brews will start at the top going counter-clockwise, left to right in the inventory GUI.
With the radial GUI open, the cursor can be moved to select a brew. Releasing the use key will throw the selected brew.