Don’t Put That Knife in the Dishwasher!

There are many myths and urban legends surrounding knife care on the Internet. Here's one: "Dishwashers will dull your knives." Seems like a bunch of hooey to me. How can spraying soap and water on a knife possibly dull the blade? Well, I've lately learned the answer to that, and as unlikely as it seems, it's absolutely true.

My prize kitchen knife is a Shun. Japanese made, Damascus steel, cuts like a dream—when it's sharp. Unfortunately, it won't stay sharp. A professionally sharpened knife should stay sharp for a few months, but my Shun would become a butterknife in a month or less. What could cause the knife to dull? 

I decided to find out. First, I sharpened my Shun to a razor edge. The photo on the left is the Shun after sharpening. Smooth, even edge with no roughness or pitting. Next, I used it for my everyday cooking, being careful not to damage it. After each use, it went into the dishwasher.

Fast forward two and a half weeks and the Shun was having trouble slicing a tomato. I looked at it under a bright light. The photos on the right are what I saw. Look at all the nicks and pits that formed in just 2 ½ weeks! Yikes!

So what could cause the damage?

1. Contact with other metallic objects? My dishwasher has a top rack with plastic slots for each utensil. My knife was always in a slot. Nothing could contact it.
2. Water? Jets of high-pressure water can cut through steel, but my dishwasher doesn't produce that kind of pressure.
3. Detergent? I use a major brand's top of the line product. I know it's caustic because it dissolves baked-on food. But could it really dissolve steel?

I can’t say for sure, but I do know this: something in the dishwasher is damaging the fine edge of the knife, resulting in not staying sharp for long. I don’t really like the lesson, but it is clear — wash sharp knives by hand and never put them in the dishwasher.

[Update] It’s been several months since I penned this, and I can say without any doubt that it is true. I’ve repeated the experiment several times using different knives, with the same result each time. High-carbon blades like my Shun are most vulnerable, which is why it happened so quickly in my experiment. But while stainless steel knives take longer to show the effects, over time the dishwasher dulls them too.