Dishwasher-safeness claims are often fibs: Free Advice #001

Remember… free advice is worth every cent you’ve paid for it.

Rule of thumb: If it’s not all-metal (e.g. a plain stainless steel saucepan) or all-ceramic (e.g. an unadorned coffee mug) or a solid lump of plastic like a one-piece, you perhaps ought to hand-wash it. Dishwasher detergents are less sudsy but significantly more chemically aggressive than the liquid stuff one uses when manually cleaning dishes. And a modern dishwasher heats water to higher and less-high-but-still-hot temperatures at different times during a run, so there’s thermal cycling going on too. Time is also a factor. A dish is subjected to a much more prolonged ordeal (chemical and thermal stress) in a dishwasher than when it’s picked up, scrubbed by hand with sudsy water at a human-tolerable temperature, rinsed, and set aside to drain. Partly in order to comply with government-imposed regulations around energy and water usage and/or home appliance ratings schemes of the same bent, currently available dishwashers need to run longer than dishwashers of yore. We’re talking hours, potentially.

Yes, my rule of thumb is dumb and full of holes. Different pieces of cookware and tableware will fare differently. Ceramic coffee mugs, for example, are generally safe, but designs on some ceramics or some elements of those designs may fade away or bleed. Others will exhibit no discernible degradation and remain mint even after hundreds of washes. And I implied that wholly-metal stuff would be fine when run through a dishwasher, but that’s not true, either. The lye (sodium hydroxide) in residential dish soap formulations turns aluminum black. Aluminum cookware and kitchen utensils are less common than they perhaps once were, due to health concerns, but some tools, like hammer-style meat tenderizers, still seem likely to be made of cast aluminum. Things like cherry and olive pitters are also still often made of cast aluminum parts. Even familiar, reputable brands that don’t offer pitters made of aluminum do sell you ones with at least the pokey bit made of zinc (i.e. galvanized steel). That’s not dishwasher-safe, either, no matter what the manufacturer may claim. Stainless meat-tenderizing hammers and pitters do exist but you may have to look a bit harder and possibly email manufacturers or ask around online or IRL.

Plain glass food storage containers? If they’re oven-safe, they’re dishwasher-safe. But what about the containers’ lids, often made of polycarbonate with silicone seals and air vent plugs? Them, not so much. The silicone parts would be fine but the polycarbonate will (ask me how I know) develop spiderweb-like crack patterns. Moreover, if the lid isn’t one solid piece (i.e. if it’s got a polycarbonate lip attached to the underside of the polycarbonate lid to help retain the silicone seal), the polycarbonate pieces will come apart after a few rounds of trial-by-dishwasher.

How about knives? If it’s a solid metal (err… by which I mean stainless steel) knife, you can chuck it into the dishwasher worry-free. The same may go for an all-plastic knife like a lettuce knife, if it’s marked as dishwasher-safe. A ceramic knife that doesn’t have a non-ceramic handle is an exotic beast, but they exist. There are also some ceramic knives with weird silicone bumpers that fit onto the edges of their large tangs that would probably also survive repeated runs through a dishwasher. Those are good to go, too. Knives with wooden handles are obviously a no-go because of the wood. Knives with plastic handles or metal knives with handles made of a different type of metal (e.g. a stainless steel knife with an aluminum handle) are likewise hand-wash-only, because the handle and the blade material will expand and contract at different rates when blasted with hot and less-hot water and something will inevitably crack or crumble or work loose after a while.

For instance, consider a Kyocera ceramic knife that has a zirconia blade and a handle made of polypropylene (PP). According to the Characteristics of Fine Ceramics: Heat – Thermal expansion article on Kyocera’s Fine Ceramics World sub-site, zirconia’s coefficient of linear thermal expansion (CLTE) of 10.5 ×10^-6 K^-1. That page gives the parameter as coefficient of thermal expansion and omits the units, but a PDF linked from a footnote (Characteristics of Kyocera Technical Ceramics) is more precise and gives finer-grained values. By the by, zirconia isn’t the only ceramic material Kyocera produces, even their zirconia products come in different varieties, and there are also zirconia-alumina mixes. The PDF (mostly a couple of material properties tables) is short but makes for interesting reading.

Polypropylene, for its part, can have a range of CLTE values depending on what exactly has been added to it (e.g. glass fiber) and how it’s been processed. Here’s a web page that includes a table of thermal expansion coefficients for a wide variety if plastics, including several polypropylene-based materials (which span a range of CLTE from 2-17 ×10^-6 K^-1): Coefficient of Linear Thermal Expansion (CLTE): Formula & Values. Note #1: to see the table, scroll down until you hit the What is the linear coefficient of thermal expansion values of several plastics? heading. Note #2: Kelvin and degrees Celsius are, in this case, interchangeable but CLTE values incorporating degrees Fahrenheit are not comparable to these without conversion.

Having a high or low coefficient of linear thermal expansion in and of itself isn’t an issue in durability of a kitchen implement over the course of many dishwasher cycles. It’s the mismatch in expansion rates that makes hand-washing such an item necessary. Again, ask me how I know.

The images embedded in this post are unrelated to the text. I snapped some photos of bags of funky-flavored Lay’s potato chips while in a local ParknShop supermarket back in early March. PnS is part of one of Hong Kong’s vertically-integrated, real-estate-based business combines and represents half of Hong Kong’s supermarket duopoly (Wellcome is the other half). Taste and Fusion are PnS sub-brands