[John Gamel]

John Gamel  asked the Naked Scientists:
   
Dear Chris,

Despite the many courses I've taken in physics, chemistry and mathematics, I don't know the answer to this important question:

My wife insists that we shouldn't put our really sharp knives in the dishwasher, since the hot water and detergent will dull the blades.

Is there any scientific evidenced to support this assertion? The answer is important, since I might be wasting a lot of time washing those knives by hand.

John Gamel, MD

What do you think?

[graham.d]

I note that nobody has tried to answer this so I thought I would give you an opinion, not based on any specific knowledge. My guess is that a dishwasher could dull a knife but probably not sufficiently for you to notice. Impact of water at high speed, particularly when mixed with some of the chemicals (surfactants) used in the dishwasher powder/tablets, is slightly abrasive. This could, in theory, dull very sharp knives but nothing like as much as simply using them to cut things. Personally, I wouldn't worry about washing them in a dishwasher.

I may be slightly (only slightly) more concerned if they were proper soft steel butchers knives, but then you should sharpen these on an iron every time you use them if you want to maintain a good edge.

[Bored chemist]

I can't see a mechanism by which water would dull knives. Being sloshed about in the dishwasher so the rub against eachother might blunt them.
Neither of these will take the edge off them as fast as using them does.

[Madidus_Scientia]

In fact, washing them by hand may be more abrasive to them.

[lightarrow]

If the blades were made of ceramic material (some modern knives blades, but it depends on what exact material) I would monitor the water's pH: it shouldn't be too high because glass/sylicates, for example, could be (very slightly) damaged.

[Geezer]

I don't think it will blunt them, but it might make them oxidise. Some high quality kitchen knives are made from carbon steel rather than stainless steel. 

[fentona]

Without question, dishwashing knives does significantly blunt them.

I have numerous high carbon steel knives that on occasion have been put in the dishwasher by my wife, and it is easy to detect the dulling of the blade on use afterwards.
One of the curious aspects of this is that it is then very difficult to return the cutting edge to the blade that has been dishwashed compared to a knife that has been dulled from normal use.

To date I have not found a scientific explanation from this,
People have proposed that this is due to mechanical dulling when rattling against other metal objects in the dishwasher.
This doesn't seem plausible as my knives are too large to go in the cutlery rack and are laid flat...

Would love an answer please - I hate hand washing

[Geezer]

I'm not sure, but perhaps heat disturbs the feathering of the edge somehow. I would not have thought the temperatures were great enough to have any effect, but perhaps they do have a tendency to re-temper the steel, or rearrange the molecules at the extreme edge of the steel. A dishwasher does keep them hot for quite a long time.

Funny things knives. When you use a steel or a strop to sharpen them, you're really just aligning the extreme edge of the blade. I suppose that, eventually, you are just organizing a few molecules.

[CliffordK]

Why don't you put your razor into the dishwasher.  Run it through 10 loads, then try shaving with it 

[Madidus_Scientia]

Without question, dishwashing knives does significantly blunt them.

Without question? Everyone is grasping at straws to even come up with a mechanism of how it could blunt them, and an internet search reveals no information that the effect is even real. So whether dishwashing knives does or doesn't blunt them, It seems to be well within the realm of question.

But who uses a dishwasher for proper sharp knives anyway? You probably only use 1 in preparation for a meal, and it's not like it gets food baked onto it and is hard to clean.

[Mazurka]

Dishwasher tablets powder are actually quite abrasive - which is why most manufacturers do not recomend that you clean non-stick pans or fine glassware (or decent knives) in them, or if you do using a liquid detergent and a gentle programme. http://www.miele.co.uk/products/dishwashers/hints-and-tips/

A second effect is that if knives (and cutlery) are left in the relatively humid interior of a dishwasher (rather than being stored in a nice dry block/ drawer) they can corode, which may not be noticeable on the blade, but may affect the edge.

Of course, all knives go blunt with use and the sharper the edge the quicker it blunts...

[Lianab]

I was looking for the answer to this after so many chefs advising me not to put them in dishwashers.  I found this about hot water:   Even “”stainless steel” can rust if it is put together with steel wool into a sink. The sink and the knife blade will rust! Use a pad of brass wire (Goldilocks) a 3 M green pad or a plastic scourer to clean the steel. Avoid submerging the blade in excessively hot water as the edge (which is only one micron thick) will change its temper and become brittle. After the first stroke, the brittle edge will break and the knife becomes blunt again, needing to be re-sharpened