How do “scratch-‘n’-sniff” cards work?

Dear Cecil: The other night I went to see John Waters’s movie Polyester, filmed in Odorama, which enables you to smell certain, ah, unique fragrances periodically via a “scratch-n-sniff” card. I was hoping you might be able to explain to me the principles of “scratch-n-sniff.” How does it work and how can I make my own? I have some interesting Christmas card ideas. Patrick W., Milwaukee

Cecil replies:

I’ll bet you do, you deve. The basic S&S process, called Micro-Fragrance Coatings, was invented years ago by the 3M Company, and is intended for the transmission of happy fragrances such as daffodils and buttercups, and not some evil reek such as you and your weirdo friends are likely to come up with. Which is not to say that, in my younger days, the same idea wouldn’t have occurred to me.

Smell, to begin with the basics, is a matter of molecules dislodging themselves from a substance’s surface and finding their way into your nose. To get something’s smell on paper, its molecules, or a lab-bred facsimile of same, must be distilled into a sort of perfume (i.e., a highly volatile liquid) that is insoluble in water. Then 3M emulsifies the liquid, which essentially means they dump it into a giant Waring blender and whomp the bejeebers out of it. Since oil and water don’t mix, we end up with millions of tiny bubbles of essence suspended in liquid. By means of a magical proprietary process that 3M has sworn me never to reveal, the bubbles are then conveyed into a plastic goo, which can then be used like ink and printed onto (preferably) some sort of stiff card stock with a modified printing press. When the plastic carrier dries, the bubbles of liquid are trapped inside until you scratch (the card, I mean), whereupon they break open and become smellable.

A similar process is used to make carbonless copy paper, only the bubbles are filled with a special chemical instead of a fragrance. When you press down with your pen, the chemical is released from the back of the top copy and reacts with another chemical coated on the front of the second copy (got that?), producing the inky stuff that makes the copy.

The bubbles, you may be interested to know, are something like half a thousandth of an inch in diameter and there are roughly 50 million to the square inch, so this is not something you can whip up in the kitchen. 3M will be pleased to knock some out for you, though. All you have to do is buy enough for about 50,000 copies.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.