Measuring a flash point using an open cup method is, as the name suggests, conducted in a vessel which is exposed to the air outside. The temperature of the substance is gradually raised and an ignition source is passed over the top of it, until it reaches a point at which it “flashes” and ignites.
The flash point here will vary according to the distance between the substance and the ignition source – the height of the source above the cup. The most commonly-used open cup method is known as the Cleveland open cup (COC).