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Cutting Edge

By Robert L. Wolke
Wednesday, December 6, 2006

Christmas is coming. The proof is the exponential increase in the number of catalogues I get in the mail. One advertises ceramic knives as being superior to metal. That's news to me, but according to Google, not to the rest of the world. Any advice on purchasing? One company offers either zirconium oxide or zirconium carbide. Is one a better choice?

If you're thinking of hinting to Santa about a ceramic knife, or perhaps of playing Santa yourself, you should know some of its pros and cons.

Long after the flint cutting tools of the Stone Age and a thousand years after humans started making knives out of steel, the 20th century brought the technology to synthesize a variety of new ceramics: hard, non-melting materials made by firing inorganic, nonmetallic substances at very high temperatures. Before long, mostly in Japan, knife blades were being made of a zirconium oxide ceramic because of its exceptional hardness.

Promotional blurbs for ceramic knives claim that zirconium oxide is second in hardness only to diamond. Not true. Corundum (ruby and sapphire) is intermediate in hardness between zirconium oxide and diamond. The black ceramic knife blades are zirconium carbide, which is a bit harder than the white zirconium oxide, giving the carbide a slight competitive edge, so to speak.

What makes any ceramic knife better than steel? Because of its extreme hardness, the edge of a ceramic blade doesn't wear away, so it stays sharp much longer than a steel blade does. It is also stainproof, rustproof and lighter than steel.

But hardness is not the same as strength. Unlike steel, a ceramic blade is rigid and inflexible. Using it for prying, twisting or pressing can simply snap it in two. (Don't use it for crushing garlic, for example.) Because the blades' edges can chip, the knives are intended only for slicing boneless and unfrozen foods. And when they eventually do become dull, they can be sharpened only by a professional with a diamond wheel.

Have your doubts? So did I. But while doing research for this column, I fell madly in love with a Kyocera six-inch ceramic Santoku chef's knife. It was so astoundingly sharp that I could cut a sheet of paper in two without even having to draw the blade across the paper's edge.

I absolutely must have one. Are you listening, Santa?

My question is about butter crocks, which seem to be enjoying a resurgence. How long can you safely keep butter in them? As you use up the butter, must you add enough water so the butter is always in contact with the water? Is it critical to start with very cold water?

Butter crocks, known also as French butter dishes or by the brand name Butter Bells, have indeed been resurging. They go back to pre-refrigerator days, when the livin' was easy and butter was always soft and spreadable.

Keeping butter at room temperature for long periods of time invites spoilage. Because oxygen in the air is primarily what causes rancidity, we could keep our room-temperature butter under water. But just try to cut off a piece while it's floating!

The French butter dish consists of a bell-shaped lid that you firmly pack with soft butter. The lid is then placed on a crock containing cold water, which seals off the bottom of the butter from the air. It can then be kept at room temperature for as long as a few weeks without turning rancid. The water, of course, will not remain cold, so don't be tricked into thinking it cools the butter for more than a few minutes. You still have to keep the crock in a cool place, because butter may melt if the room temperature reaches 80 degrees.

But there's always a catch, isn't there? Or several. The water must be changed every few days to prevent mold, which can form in warm climates and certain other environments. And as soon as you use some of the butter, its volume is replaced by a pocket of evil air, not by water. More added water cannot fill that pocket because there is no place for the air to go. It just stays there, in contact with the remaining butter, until you refill the bell to the brim. Thus, the protection from air isn't complete as the butter is used.

If it were up to me, I would make the crock out of thin, unglazed, porous clay, unlike those porcelain beauties you see in the stores. The outside of my porous crock would always be damp, and evaporation would keep the butter a few degrees cooler than the room.

Why not just keep a few days' worth of butter in a covered butter dish on the kitchen counter for both spreading and cooking? It's not going to turn rancid that quickly, especially if it is salted.

Robert L. Wolke (http://www.robertwolke.com) is professor emeritus of chemistry at the University of Pittsburgh. His latest book is "What Einstein Told His Cook 2, The Sequel: Further Adventures in Kitchen Science" (W.W. Norton, 2005). He can be reached at wolke@pitt.edu.

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