What should never go in a dishwasher?

A dishwasher is a much harsher environment than a sink: detergent at pH 10-12, water at 50-70°C for hours, and jets driving it into every crevice. Almost everything on the do-not-wash list fails for one of those three reasons rather than because of some special rule, so the principle is more useful than the list.

Step by step

1. Keep out anything cast iron or carbon steel.The seasoning is polymerised oil bonded to the metal, and alkaline detergent strips it in one cycle. Underneath is bare iron that rusts within hours in a wet tub. Wash by hand, dry on the hob, wipe with oil.
2. Keep out wood, including bamboo and wooden handles.Wood absorbs water, swells, then dries unevenly and splits. Glued boards delaminate. A wooden handle on a knife or pan will loosen and crack. Bamboo boards are laminated and fail fastest of all.
3. Keep out good knives.Two separate problems. The detergent dulls and pits the edge and can stain the blade, and the blade knocks against the basket and other items. A chef's knife takes thirty seconds to wash by hand and lasts decades longer for it.
4. Keep out non-stick pans, whatever the label says.Many are marked dishwasher safe and will survive for a while, but repeated alkaline cycles degrade the coating and its bond to the pan. This is the fastest way to shorten the life of a non-stick pan.
5. Keep out aluminium, copper and pewter.Aluminium oxidises to a dull grey or black in alkaline water — a chemical reaction, not dirt, and it does not come off. Copper loses its polish permanently. Pewter is soft and can distort at dishwasher temperatures.
6. Keep out insulated flasks and travel mugs unless they say otherwise.The vacuum seal between the two walls is the whole point of the item, and heat cycling plus water at the seal ruins it. A flask that no longer keeps drinks hot has usually been in a dishwasher.
7. Keep out hand-painted, gilded and antique china.Over-glaze decoration and any gold or silver band sit on top of the glaze rather than under it, and the detergent removes them. Once a gold rim has gone patchy, nothing brings it back.
8. Keep out crystal and very thin glass.Lead crystal is softer than ordinary glass and etches quickly. Thin glass suffers thermal shock and chips against neighbours. If it matters to you, wash it by hand.
9. Keep out anything glued or with a printed label.Glue lines soften, handles come off, and printed measurement markings on jugs wash away. Melamine crockery and anything with a decal applied after firing belongs in the sink.
10. Be careful with the things that are borderline.Stainless steel is fine but goes spotty without rinse aid. Silicone is fine. Plastic is fine on the top rack only. Baby bottles and teats are fine on a hot programme but check the manufacturer's guidance. Anything with a rubber seal will outlast the machine if hand washed and will not if not.

Tips

• Dishwasher safe means it will survive, not that it will look the same in five years. It is a durability claim, not a quality one.
• Stainless steel and silver must never touch in a dishwasher. The two metals in hot detergent solution set up a galvanic reaction that pits the silver permanently.
• If something has come out dull, grey or discoloured after one cycle, it is a chemical reaction with the detergent. It will not wash off and it will get worse each time.

Common mistakes

• Trusting the dishwasher-safe symbol on a non-stick pan — The symbol means the pan will not fall apart. It says nothing about how long the coating lasts, and the coating is the only reason you bought the pan.
• Putting a cast iron pan in once, just this time — One cycle is enough to strip the seasoning completely and start surface rust. Re-seasoning is an hour in the oven and several coats — considerably more work than washing it would have been.

Questions people ask