HOW COME? From log to petrified wood

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How does ordinary wood turn into petrified wood, which feels like rock? asks a reader.

A petrified log may look like an ordinary wooden log, complete with growth rings and bark. But touch it, and you'll realize that the log is like a model of the original, cast in rock. The stone log may also be streaked with vivid colors and glitter in the sun.

Think of a chunk of petrified wood as a fossil, like a dinosaur bone. Most wood ends up disappearing over time, decaying and disintegrating, burned up by fire, or eaten by bacteria, insects or animals. But wood that has petrified escapes this fate, transforming into a rocky replica of itself.

For a chunk of wood to petrify, conditions must be just right. For example, a log floating down a shallow stream becomes trapped in volcanic ash or mud. The oxygen-free environment inside the muck helps preserve it. And over time the buried log's cellulose cells gradually lose their fluids.

Later, mineral-rich water from outside seeps into the mired wood's empty cells. When the intruding water itself evaporates, minerals remain behind. Silica, calcite and other minerals fill the wood's cellular nooks and crannies. As the minerals grow and harden over hundreds of thousands of years, the original log is replaced, cell by cell, by rock.

Petrified wood comes in a rainbow of colors. Iron, copper, manganese and other elements in water add reds, yellows, blues and greens. Crystals of quartz and other minerals make petrified wood glitter. In once-hollow logs large gemstone crystals such as amethyst or rose quartz may slowly grow, filling the space over millions of years.

In 2005, researchers in Washington state announced that they had created petrified wood in a lab -- in mere days. The scientists went to a lumber yard and bought some ordinary poplar and pine boards. They cut the wood into 1-cm (.4 inch) blocks. The little cubes spent two days stewing in a caustic acid bath, and another two soaking in a silica solution.

After the blocks dried, they were placed in a special furnace full of argon gas. (Argon, it turns out, works better than plain air at growing certain crystals.) The cubes were baked for two hours, at a toasty 2,552 degrees F.

Afterward, the blocks, still enveloped in argon, were cooled to room temperature. The result? The silica had combined with carbon in the wood's cellulose fibers, creating a hard ceramic called silicon carbide. Presto: petrified wood.

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