The dishwasher safe symbol is one of the trickiest food safety symbols to interpret, and for good reason: there’s no officially approved symbol that signifies an item is dishwasher safe!
What’s more, the phrase “dishwasher safe” isn’t a standard claim either. Each company producing kitchenware or tableware has their own process and standards for determining what can be called dishwasher safe, based on their own methods and materials.
Generally speaking, manufacturers will say an item is ‘dishwasher safe’ if they’ve treated it at some stage of the production process to ensure it can withstand both the prolonged high temperatures and the detergents involved in a dishwasher cycle. There are different techniques that manufacturers use to ensure their products are dishwasher safe, with methods varying depending on the material in question. We’ll delve deeper into these techniques a little later on.
Dishwasher symbols explained
Typically, there are two types of dishwasher safe icons:
- Dishwasher safe, top rack only
- Dishwasher safe
If an item is marked ‘top rack only’, it means it’s only safe to be washed on the top rack of the dishwasher. The heating element in dishwashers is typically on the bottom of the machine, which means the top rack is comparatively cooler than the bottom. If you put an item marked ‘top rack only’ in the bottom rack, you may find it has discolored, warped or become damaged by the end of the cycle!
For both types of dishwasher safe items, typically the dishwasher safe symbol looks like a square box with some plates or glasses (or both) inside it. You’ll also see either water droplets or diagonal lines that are meant to signify water. For items that are only ‘top rack only’, the droplets or diagonal lines will only hit the top of the plates or glasses. If the symbol just shows droplets or lines that cross the entirety of the square box, it’s probably safe to wash the item on either the top or bottom rack.
The image may also include a temperature to clarify the safest top temperature you can wash the item in. Sometimes wording will accompany the symbol, which helps eliminate some of this symbol-deciphering guesswork!
How manufacturers make items dishwasher safe
As we’ve mentioned already, manufacturers use different techniques to make their items ‘dishwasher safe’. Below we cover some of the main materials you’ll find in your kitchen or home, and how they are treated for that dishwasher (unofficial) seal of approval:
Dishwasher Safe Glass
Most glass is ‘annealed’ which means that when it is made, it is slowly cooled to even out any stresses within its structure. It’s a routine process that almost all glass products go through, and helps make ensure any residual internal stresses are relieved. Annealed glass is often called ‘untreated glass’, because there are two other processes that glass can be put through to make it more resilient to drastic temperature changes.
Glass can be put through either a process called ‘tempering’, which is a thermal or chemical process that makes glass tougher and stronger, or a milder form of tempering known as ‘heat-strengthening’. Kitchenware is usually heat-strengthened, and this makes it twice as resilient as annealed glass. You may find that some cookware is fully-tempered, making it four to five times stronger than annealed glass, and able to withstand microwaving.
Dishwasher Safe Ceramics
Ceramics are made dishwasher safe through a similar process as glass: slow cooling that allows the material to settle with as few imperfections as possible. In short, a glaze is applied to ceramics to protect the surface. The type of glaze used can dramatically affect the strength and durability of a ceramic item. Always check for the dishwasher safe symbol before putting your ceramic items in a dishwasher.
Dishwasher Safe Plastics
Plastic items typically don’t need to be ‘treated’ to make them dishwasher safe because the type of plastic used to make the item often determines whether or not it is dishwasher safe. Plastics with a very low melting point can melt in the high temperatures of a dishwasher. Only place plastics in the dishwasher if they are clearly marked with a dishwasher safe icon and stick to the top rack when you do so.