The approximate dimensions of trays are the following:
Small: 1.7x2.2 inches
Medium: 2.1x3.5 inches (fits the 'Altoids' tin)
Large: 3.1x4.3 inches
All mixing trays are made of white compostable bioplastic (details below). They are sturdy and designed specifically to fit inside the metal travel tins. When not in use, they can be stored inside the tin, on top of paints (if you use magnets with your pans, some tin boxes may not fit both magnets and the mixing tray when closed; check the tin box height before adding magnets or the tray or consider storing the tray out of the tin box). The trays are easy to wash or wipe after use. If you use staining watercolors or ink, you will need to use dish soap, laundry detergent, "The Masters Brush Cleaner", Clorox Foamer, toothpaste, or a week solution of Clorox with water. Use soft sponge or and/or soak the trays until they are clean. Do not use harsh chemicals (rubbing alcohol acetone, ammonia), they will destroy bioplastic. Do not use abrasive cleaning supplies - if the surface is scratched, it will pick up more staining particles next time you use the staining media and it will be harder to clean.
There are four tray configurations for each tin size: solid flat with borders but no sections inside, so that colors can travel anywhere on the mixing tray, and with two, three, or four sections, if you want to keep color mixes separate (for example, background and foreground colors, or warm and cool colors). Depending on your style of painting and mixing, you can pick the tray that best suits you. Please see photos!
What is compostable bioplastic? Compostable bioplastic is made of micro-organisms and renewable raw materials, such as, corn, cassava, sugarcane, or sugar beet pulp. Some variations of this bioplastic may include wood. Although this type of plastic still requires recycling, as it can only be composted in an industrial environment, it is still a better alternative to common non-biodegradable plastic that requires the use of nonrenewable petroleum and causes pollution.