The mystery of the rusty ceramic knife (SOLVED), or destructive partial disassembly of a Kyocera ceramic santoku knife

Last time (Dishwasher-safeness claims are often fibs: Free Advice #001), I mentioned that our older ceramic knives’ plastic handles had developed cracks and propounded my hypothesis that the cracking was a consequence of the blade material and the handle material having different coefficients of linear thermal expansion and my having, in the past, run them through our dishwasher periodically.

The aforementioned cracks are difficult to make out in the photo above, so here’s a composite image in which they’re more readily visible:

There are three cracks. Four if you count the other, still-closed, crack on the top of the handle. Two are on either side of the bottom of the handle and the other one (or two) are on the top. In these close-up views (click on the photo for a larger version or right-click and open the image file alone for a full-sized view), the cracks seem to be edged with a bit of a reddish/brownish/peanut-butter-colored material. I tried probing the cracks with a dental pick and didn’t dislodge any gunk.

That made sense because I’d switched to manually cleaning (i.e. hand-washing) this and our other plastic-handled, black-ceramic-bladed kitchen knives years ago and had been, when they periodically needed cleaning, using a soapy dish sponge on the handle and blade and a soft-bristled toothbrush and soapy water to clean the area on each of these knives where the blade emerged from the handle. So I’d have been nonplussed if I’d found food particles in those cracks.

The cracks in this knife and the in the others of the same type which we’d purchased around the same time had begun forming in their dishwasher era but they’d continued to develop and had grown in the years since. Even so, the blade remained firmly seated in the handle, not loose at all, and I still used this knife from time to time. But an attack of irresistible curiosity struck.

There was a slight but real gap between the handle and blade, at least on the no-logo side of the blade, large enough to accommodate the business edge of a small pry tool. I set to work with that spudger and a pair of small duckbill pliers.

See the corrosion? While I was butchering this knife, rust particles rained out of the handle and off the blade tang. The photo above was taken against the backdrop of a fresh, spotless paper towel. Both faces of the stump of the tang were originally blanketed in a thick layer of powdery rust, though here it’s been thinned out on the visible side and crisscrossed with furrows by my efforts with the prying implement and plier jaws.

Plastic doesn’t rust. Zirconia doesn’t rust. So what’s rusting? Here are some zoomed-in snapshots of the same surfaces:

Hidden away inside the handle, on opposite sides of the blade tang, were two flat pieces of perforated ferrous metal. Or else it’s one strip folded over beneath the buried end of the blade. Unless I bring a powered cutting tool or more force (applied via a hammer and chisel) to bear, I may never know for certain.

Perhaps the metal is present to lend extra rigidity to the handle. Or to increase the overall heft of the knife. Or for compatibility with magnetic kitchen knife racks. Or perhaps to ensure the knife registers if someone attempts to, for example, carry it through a metal detector. I’d never brought a magnet anywhere near one of our ceramic kitchen knives until after I’d mangled this one (result: the magnet sticks to the handle, or vice versa) and so had never suspected the presence of concealed bits of ferrous metal.

The odd appearance of the lips of the cracks also makes sense in light of this little exercise. The blackness of the handle material is merely skin-deep and the bulk of the plastic is a creamy, off-white hue and that, tinged with rust, was being exposed along the edges of the fissures.

Packaging and ancillary materials associated with our plastic-handled Kyocera knives went into the trash ages ago and the pictures above were taken of one of the pamphlets included with each of our recently-purchased Kyocera knives. All of the new arrivals are from the line with riveted-on wooden handles, but the advice inside addresses both types. One of the do-not-do illustrations shows a dishwasher and the Kyocera technical writers are very clear on the necessity of hand-washing knives with wooden handles. When it comes to knives with plastic handles, however, the pamphlet states Kyocera ceramic knives with plastic handles are dishwasher-safe and instructs owners to place them in their dishwashers’ top racks in such a way as to keep them out of range of metal implements. Which is precisely what I used to do.

Though it still seems like a no-brainer that the handle cracking in these knives could’ve begun as a direct consequence of the differential rates of expansion and contraction of the ceramic and the plastic (and also of the embedded metal strip[s]) during however many trips our black-plastic-handled knives took through our dishwasher. But there’s a role there for the rust as well. Metal corrosion occupies more space than un-corroded metal and the rust had continued gradually cranking the plastic handle of this knife apart even after I went to manual washing. Even if I’d never put them through a single dishwasher cycle, water ingress was inevitable and the ultimate result would have been similar.

And if there is a vital need for embedded metal strips, why doesn’t Kyocera opt for stainless steel? If being magnetic is a hard requirement, they still have choices. Stainless steel alloys that are both magnetic and resistant to corrosion do exist.

We’ve arrived at what feels like a good place to wrap this post up, but I have a bit more to share about ceramic knives. While I was looking online for discussions of cracked handles on ceramic knives, I stumbled across some discussion of ceramic knife blades chipping through normal, as-prescribed-by-the-manufacturer use and handling. See, for example, the image of a Kyocera black ceramic knife edge included in Ceramic knives – Why we don’t sell them at knivesandtools.com.

Our new knives, the ones with the wooden handles, are perceptibly sharper than the previous generation, the ones like the plastic-handled 5.5-incher I deconstructed. But those old ones still seemed to be perfectly usable and we hadn’t stopped employing them in our kitchen. A casual glance at the blade edge with the naked eye didn’t turn up any alarming-looking deterioration.

Curious again, I affixed a clip-on magnifier (billed as a 200x-magnification microscope) to a phone, set the liberated blade from the old knife on some 1mm-square graph paper, and took some pictures and video. Then I did the same with one of the new knives.

Yeesh, the old blade looks rough. But there’s a tiny nick on the new knife’s edge too. These video clips show more of the edges than would be visible in single photos:

I was less successful at keeping the cutting edge of the new knife in focus, as you can readily see below:

Its still-intact-and-still-attached handle wasn’t well-supported in my makeshift videography setup and the handle’s weight was cantilevering the blade up towards the camera. As a result, it was elevated further from the graph paper background, which wound up being further from the lens (and hence less distinct-looking). How much further? You can get a qualitative feel for the difference by eyeballing how much smaller the squares of the grid appear in this video than in the previous footage.