McCormick adds a dash of salt, cardamon, basil, thyme. . .

This may be the spiciest story you'll ever read in this newspaper. It's about the the largest producer of spices in the world, McCormick & Co., a $660 million-a-year business that has made several fortunes from processing spices that look like garden mulch in their natural state.

A tour of the dowager empress of the McCormick factories, the Light Street plant on Baltimore's inner harbor, shows how the company transforms raw spices into international seasonings.

You can sniff the factory almost before you see it - a whiff of exotic and indefinable pungencies mixed with the salt air of the harbor. Then the nine-story, banana-color building bulks into view.

It sits like a great square yellow buoy on the edge of the waterfront, a venerable landmark surrounded by the jazzy contemporary architecture of Baltimore's recently renovated harbor. To its left: the shimmering, abstract-looking Hyatt Hotel, an angular dome sheathed in blue-gray glass that reflects water and sky. Beyond it: Harborplace, the multimillion-dollar recreational shopping plaza inspired by a Victorian pavilion. Just across the water: the concrete-and-glass whale of the National Aquarium building, with its mosaic of brightly colored panels.

Inside, the McCormick Building is a different world. It is a quaint, period world, rather like a sepia movie in ''smellovision.'' A rich, strong scent perfumes the air, an elusive scent that seems a mixture of pepper, cinnamon, mustard, and cloves - but with subtle undertones, perhaps of cardamom. It envelops you as you enter, permeates everything, and clings to your clothes and hair for hours.

''Smell? I don't smell anything,'' says tour guide Donna Stout, who admits that, after a while, people become impervious to the scent.

A journalist's tour starts at the top, the seventh floor, because of the way many of the spices and herbs are sorted, sifted, cleaned, and funneled into their final form. ''Gravity pulls the products through'' to various stages on the lower floors, Mrs. Stout explains. In a vast room with windows looking out over the harbor, we see hundreds of bales of brown burlap bags. ''Product of Turkey'' is stamped on one heap, and sticking out of a split corner are elliptical, crisp, olive-green bay leaves.

Another series of sacks, from Indonesia, holds reddish-brown, curled cylinders of cinnamon bark. There is a bright-orange dust on the floor underneath the red pepper. The fine, red-pepper dust, like Sahara sand, filters into nose, mouth, and ears, until you feel you're trapped in a giant pizza. In this room, products are sifted and aspirated (blown into the air) to check for impurities.

Bypassing the sixth floor, where the spices are simply dumped down to the fifth, we work our way through the cleaning, filling, and grinding center. Here, tiny stems, rocks, and bugs spotted by the quality-control laboratory are removed by milling machines. Spices are sent to various machines, depending on the type and amount of impurities they may contain. Each machine, Mrs. Stout tells us, is good at doing one or two specific things; some are best for removing sticks and stones, others for extracting insect fragments and hair. We watch the tiny cream-color seeds of fennel, which resemble bird seed, sluicing down into separate funnels on a gravity table. Our guide picks up handfuls to show us the difference in grittiness before and after this process.

The room is full of bizarre-looking machines that might have been designed by cartoonist Rube Goldberg. The gravity machine, for instance, resembles an electric milking machine without the cow. There is a terrific din as the machines whir, tweet, vibrate, shake, and spin away in their metallic fashion. On the separator rollers, black peppercorns are sorted; they fly out into the air like pungent little BBs. Hard, brown pellets of nutmeg careen back and forth on the flat-bed sifter.

In another area, the poppy-seed sifter is busily separating a pale-tan residue from the silky gray-blue seeds. The crushed-red-pepper tailings are shoveled up, but they fall like thick shavings from a mammoth red pencil. We learn that pungent yellow saffron, precious as platinum, is kept in a safe. It sells wholesale for $700 to $900 a pound.

The fourth floor is devoted to ''catching off,'' which is simply catching the products from the upper floors in drums or bins. The products, at this point, have been through an herb-and-spice marathon: a visual check, involving ''heavy-filth tests,'' ''light-filth tests,'' and an ''extraneous-matter test'' - industrialese for the screening process that guarantees the products' purity.

Constant communication goes on between the fifth-floor milling area and the quality-control laboratory downstairs. On an automatic ''writer,'' printed messages are sent between the two points. A nutmeg note reads ''sieve acceptable ,'' and adds ''this is running coarser than usual.'' The whole Turkish oregano notation is ''sieve is acceptable, heavy filth is all right.'' It means the test is all right, not the filth, which has been screened out. There are whole bound-leather notation books on the various spices and herbs, inscribed with titles like ''Chili IV'' and ''Onion III.''

''We're running a bunch of cinnamons'' the guide, stepping into an aromatic cloud of them in the quality-control laboratory. The cinnamon samples are mixed with water and a solvent, then distilled in what looks like two-dozen coffee pots to measure the volatile oils of each. Nearby are the other checking areas: the chemistry, microbiological, and extraneous-matter laboratories. In terms of processing, says Mrs. Stout, ''the most difficult is the marjoram, particularly from Egypt, because it has a lot of dead insects. . . . The easiest to deal with is black pepper.''

Although the McCormick company keeps a buttoned lip about how many pounds of various spices and herbs pass through this factory annually, it is safe to say that the totals are in the hundreds of thousands of pounds. There are also no figures on how many pounds of spices the firm imports annually, but spokeswoman Eileen Keogh cites some national import statistics: 48 million pounds of mustard a year, 47 million pounds of sesame, 33 million pounds of black pepper, 6.5 million pounds of red pepper, 3.5 million pounds of ginger, and 3.5 million pounds of caraway seed.

Miss Keogh said that during the last year for which figures were available, 1981, there was a 10 percent increase in spice consumption in this country over 1980. In the past 10 years, she explained, there has been a dramatic increase in the use of spices: basil consumption has increased 460 percent, fennel use is up 150 percent, anise up 110 percent, cumin up 102 percent, and sesame up 84 percent.

The general public tour does not include the same inside look at the factory given a visiting reporter because of Occupational Safety and Health Administration regulations, company officials say. The public tour begins with a McCormick movie, ''The Wonderful World of Flavor,'' in an art-deco theater decorated with murals of the spice trade. One sees the orchids from whose seed pods vanilla flavoring comes, and learns that the Visigoths demanded 3,000 pounds of pepper for the ransom of Rome, among other piquant facts.

The company's Friendship Court, is a little slice of Elizabethan life that includes a replica of Anne Hathaway's thatch-roof cottage, Shakespeare's grammer school at Stratford-on-Avon, and a 16th-century grocery store. Tour guide Dotty Hutchins, who is wearing a full-length saffron-print Elizabethan costume, leads into Ye Olde McCormick Tea House. It is a Tudor replica of an English country inn from the era of Charles II with beamed walls and ceilings, baronial tables and chairs, and a fireplace with a baking settle for courting.

Then it's on to the Tea Museum, to see a model ship made entirely of cloves (complete with outriggers) by a 10-year-old Indian boy, various other artifacts, and the room-high ''Book of Tea,'' entirely covered in cowhide, the biggest book in the world, according to ''Ripley's Believe It or Not.'' The tour also includes the antique office of company founder Charles P. McCormick, the board room, and finally the test kitchen, a lemon-color room where staff members test McCormick products against those of their competitors.

The Light Street plant is one of two primary spice-processing plants in the McCormick empire, which has 58 other food-processing facilities around the world. Company products are sold in 85 countries, from Japan to Venezuela. Seasonings and flavorings represent 65 percent of sales, but McCormick is also involved in real estate and packaging.