Dryer Up.

It all began with a tantalizing British problem: Shipments of U.S. tanks arrived in British ports all assembled and tuned up. But before they could be driven away, they had to be "de-slushed" of heavy coats of rust-preventing grease. While they lay helpless at dockside, Nazi bombers could knock them off like sitting ducks.

The British appealed to U.S. shippers: was there no way to pack the tanks so they could be driven off pronto? The next shipment of tanks came wrapped in plastic film, with a drying material inside to prevent rust. The British simply ripped off the wrapping and roared away.

By last week this useful packaging discovery had become a prime aid in the shipment of U.S. equipment, from airplanes to bomb sights, all over the world. The process, which has big postwar possibilities, was exhibited at an ordnance show in Philadelphia. The nub of the matter is a substance called silica gel.

Adsorbing Gel. One of the most efficient adsorbent materials known, silica gel was first produced commercially (for use in gas masks) in World War I. It also has industrial uses as a dehydrator and catalyst. Made by drying a gelatinous form of silicon dioxide, silica gel looks like crushed quartz, is riddled with invisible pores so numerous that a cubic inch has more than 50,000 square feet of interior surface. By adsorption (sticking of moisture to the surface), silica gel can hold half its own weight in water without swelling, caking or developing a visible sweat.

In packaging, bags of silica gel are enclosed with the packed object in a vapor-proof wrapping like Pliofilm or reinforced Cellophane. To show whether the air inside is keeping dry enough, the silica gel is impregnated with cobalt chloride, which turns pink if humidity rises above 30%--the point at which metal begins to rust. After unwrapping, silica gel can be dried and used again.

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