Dishwasher safe is a phrase that you’ll probably recognise, and will have seen on the bottom of some glasses and plates. You probably won’t know exactly what this means however, and for good reason: dishwasher safe is not a standard label or claim! Each company producing kitchenware has their own process and standards for determining what can be called dishwasher safe, based on their own methods and materials.
What it means
Dishwasher safe essentially means that the item has been treated at some stage of the production process, to ensure that the prolonged high temperatures of the dishwasher, and highly effective detergents, don’t compromise the items and cause them any damage. This works both ways of course, as attempting to wash an item made of a thermoplastic (a plastic with a low melting point) could cause some real damage to your dishwasher.
How manufacturers make items dishwasher safe
For different items of different materials, there are different techniques that are employed to make the product ‘dishwasher safe’.
Most glass is ‘annealed’ which means that when it is made, it is cooled slowly to even out any stresses within its structure, making it more equipped to deal with temperature changes. Glass can also be put through a process called ‘tempering’, which uses a thermal or chemical process to make it much tougher and stronger. Kitchenware usually undergoes a milder form of this known as ‘heat-strengthening’ which makes it twice as resilient as annealed glass, although some cookware may have been fully-tempered, making it four to five times stronger than annealed glass1, and able to withstand microwaving.
Ceramics are made dishwasher safe through a similar process that glass is, through slow cooling that allows the material to settle with as few imperfections as possible. There are several differences between the two, most importantly in the materials that are used to create them, but when it comes to making them dishwasher safe, a glaze is applied to ceramics to protect the surface – the type of glaze can dramatically affect the strength and durability of an item.
Plastic is not typically ‘treated’ to make them dishwasher safe, rather the type of plastic that an item is made from determines whether or not it is dishwasher safe. If a plastic has a very low melting point then the high temperatures of a dishwasher can cause it to melt, but other types of plastic could become brittle and crack.
How you can tell if an item is dishwasher safe
Because of the non-standard nature of dishwasher safe claims, different producers of kitchenware may use a different symbol to show that it can be used in a dishwasher. This will most commonly look like a plate or a glass with water drops above it, but will sometimes be accompanied by the label ‘dishwasher safe’.