Photo Emulsion is a chemical used for screen printing with the "light exposure" method, which yields a stencil that can be either temporary or permanent depending on how you treat the screen following your print job. Photo emulsion MUST be activated with the help of a sensitizer (which, as the name suggests, makes the emulsion light sensitive.) This 26.4 oz jar pairs with Speedball's 2 oz Sensitizer for an exact mixture ratio that can then be stored in a refrigerator for up to 4 months. Make sure you have sensitizer when purchasing this, as this jar is only the inactive emulsion! If you're new to screen printing, consider getting the full Photo Emulsion Kit which includes emulsion, sensitizer, and detailed instructions.