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What is it about the dishwasher that harms knives?

The only reasons I've found are:

  • You could get cut... I personally have a greater change of getting cut by trying to wash them by hand.
  • The blades can damage the plastic-coated metal shelves... so I put them in the silverware-holder instead of on the rack (it's 100% plastic, no metal).
  • The blade could get nicked by being knocked into other silverware... so put the blade by itself in a section of the silverware-holder in the DW.
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I definitely think nothing wooden should go in the dishwasher, b/c the wood will eventually crack. But what about knives with no wood? – JustRightMenus Jul 18 '10 at 19:19
I've never heard of anyone not putting knives in the dishwasher. I've always put them in and they're fine. – MGOwen Oct 5 '10 at 3:10

10 Answers 10

up vote 11 down vote accepted

Expensive knives can survive a trip through the dishwasher, but like others have mentioned, they can bump into things, end up with coatings of detergents and such, suffer damage to wood, etc. Why would you do that to an investment?

Cheaper knives will just straight out rust/corrode, even if you remove them and dry them right after the cycle. Yeah, you can scrub it off, but you're also losing metal from the blade, possibly starting rust in areas you can't dry, and generally degrading the knives.

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Well even for knives with no wood, a dishwasher is a very hostile environment. The reason is primarily for the blades. If you have quality knives that you care for, and plan to keep for many years, then it's just not worth it. It's just too easy for a knife to be jostled around and bang into other knives or silverware and get nicked.

You mention that you have a greater chance of getting cut when washing by hand. Well, there's a trick to that.

The easiest and safest way to wash a knife by hand is to press it flat against the side of the sink, then use your sponge/scrubber on the exposed side of the knife. Repeat for the other side. This keeps your blade safe, because the edge never touches anything. It also keeps your sponges and hands from being slashed to pieces by the blade.

Dry it immediately with a dish towel, using a pinching swipe from bolster to tip from the back of the knife.

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Main focus on cleaning any knife without getting cut is really as you write...from the back of the knife. I've never cut myself while cleaning my knives, the only people I know that have done that are very clumsy of just the "kind" of people that throw all the knives into the sink and grabs randomly into the deep with allot of bubbles on top of their water :) – cyberzed Jul 18 '10 at 19:47

I've heard that certain components of dishwasher detergents may actually be so agressive you run the risk of making your knives dull. I'm not an expert on metal <> cleaning agent interaction, though, so kindly don't take that as an absolute truth.

What I have observed is that just putting the knives in the dishwasher loosely (as you oftentimes have to do due to the length of the knives) tends to have them rolling about a bit during the cleaning process, which makes them bump into other (metallic) objects such as pots and pans. That will make them dull, and that is why I always wash my proper knives by hand.

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The biggest reason I've heard is that the wood handles can't survive in there and still look good. It can still work, but you have to be vigilant to remove the knives immediately on completion of the wash cycle. And yes, it's true. My wood-handled knives are noticeably grayer than their newer brethren.

Our non-wood handled knives (those Wüsthof blades) go in the dishwasher with nary a worry.

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The easiest way to damage any metal edge is to expose it to water and chemicals for longer than necessary. When the water just sits on the blade, the corrosion causes the metal to flake off (at a microscopic level). Immediately wash and dry your knife after use, and it will last you the rest of your life with minimal damage. I have also heard of some folks brushing rubbing alcohol along the blade to help dissipate the water.

This goes for shaving razors as well.

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There was a very interesting article on Lifehacker a while back about people who dried their disposable razors by running them backwards on jeans/a towel/their forearm - and their crappy disposable razors would last 6 months to a year in some instances. Anecdotally, since I got a few very good knives and have started drying them immediately after every use, they've retained their sharpness for an incredibly long time compared to before. – stephennmcdonald Aug 3 '10 at 15:11
@stephenmcdonald: Interesting article! Here's the link. I won't be leaving my knives in the drainboard any more. – Neil Fein Aug 23 '10 at 21:48

My knives have wooden handles which are not dishwasher friendly (the varnish has a habit of melting). I'm also suspicious of the effect of the rinse aid which seems to add a thin coating onto everything, which might not be good for the blades.

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A thin coating? I would stop worrying about the blades, and worry about what on earth it is you're digesting! – Arafangion Oct 6 '10 at 13:47

the heat and water absolutely destroy wood handles. i can't think of a faster way of ruining a good knife with a wood handle.

dishwashing agents also corrode metal - including stainless steel. i've had good knives get pitted from bits of dishwashing powder stuck to it. the problem is especially bad for carbon steel knives.

don't do it - wash by hand.

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Dishwashers will heat the blade to a temperature that changes the structure of the steel itself so it will not keep a sharp edge. It only takes one time in the dishwasher for this to happen so if yours has taken that trip through--too late. If your knife is brand new--just be hyper-vigilant that one ever puts it in.

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Interesting, which structure do you mean? I know of different types of steel structure created by heating and cooling, such as martensitic vs. austenitic steel, but they are created by cooling the metal down white-hot to room temp. Are you saying that the 60 Celsius in the dishwasher are sufficient to change this? Or do you mean something else by "the structure of the steel itself", and what is it? – rumtscho Jul 11 at 19:25

The stainless steel used for edges is not the same kind of stainless steel that is used for pots and bowls; it can be hardened far better but is by far not as corrosion resistant. Look up 300 vs 400 series of stainless steels. No one would usually make a sharp knife out of 300 series unless they are making a dive knife.

Also, especially expensive knives are not always made out of stainless steel at all, there are plenty of examples made out of non-stainless carbon steels or so called semi-stainless alloys (for example D2 aka SKD11).

Some of the cheapest blades are the most corrosion proof.

Dishwasher chemistry is harmful to all of these.

Also, thermal cycling, especially if uneven, can cause things to warp; with some blade designs warping is very bad news since they will be difficult to straighten and difficult to sharpen cleanly while bent.

Of course, handles made with wood, lacquer, bone, horn... are not dishwasher proof anyway.

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Can anyone give a scientifically solid answer than "I think so"? I read a web page which gives 4 reasons why good knives should not be washed in a dish washer. None of the reasons can be applied to my arrangement of our good knives in our good dish washer. Plus, Hand wash usually cannot clean knives thoroughly, and hand wash of sharp knives increases chances of cutting our hands.

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Hand washing can clean things thoroughly (I don't think any food safety/health agencies tell you that you have to have a dishwasher to be safe), people wash knives all the time without cutting their hands, there are plenty of good reasons given here, and the link you posted doesn't seem to have anything to do with knives... – Jefromi Jul 22 '13 at 5:59
The only part of this answer that actually answers the question is the link, and it doesn't direct me to something about knives... – Mien Jul 22 '13 at 6:26
I removed the link, as it is indeed not about knives. I will give you the benefit of doubt that you just pasted the wrong link instead of knowingly inserting an unrelated promotional link, and not delete the whole answer. But in this case, it would be best to list these 4 reasons here and explain why you think that they don't apply to your case. Until then, the answer is not very believable. Especially when you challenge others to provide proof for their opinion, but you do not give any for your own. – rumtscho Jul 22 '13 at 8:54
@Jefromi No, dishwashers are not required; most codes have specific details on setup and sanitation for handwashing. – SAJ14SAJ Jul 22 '13 at 9:08

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