THOUSANDS OF years ago some caveman or lady, as the case may be, discovered that food could be covered with wet clay from a river bank and thrown into a fire to cook. Undoubtedly some neolithic Duncan Hines commented that the result was superior to that prepared over an open flame...the meat and fish had retained their moisture. The cook loved the system because constant attention to the spit was no longer necessary. Once the clay-covered food was into the embers, everthing was done.

By the time the Romans came along, the reusable fire-resistant clay cooking vessel had become a basic kitchen tool. As a matter of fact, one of the popular brands of contemporary clay cooking pots is called a Romertopf, which is German for "Roman pot." The material from which these pots are made is similar to the clay in flower pots.

The clay cooking technique is very simple. Before you use the pot you soak it in cold water for 15 minutes. The pot will absorb an enormous amount of water. If you have a clay pot which is totally unglazed, it's a good idea to line the bottom half with a piece of parchment paper. The paper will help prevent the pot from absorbing the strong essences of foods such as raw garlic or onions. If your pot has a glaze inside the base, skip the parchment and proceed to fill the pot with your ingredients. Put the top on the pot, place it in a cold oven, and turn on the heat.

That last point is very important. If you put a moist clay pot directly into a hot oven, there's a good chance it will crack. As the heat slowly rises, the water in the clay is drawn into the pot where it blends with the juices of the food forming a steamy mist which envelopes the ingredients. It's not necessary or advisable to use cooking fats or oils in a clay pot. That is good news for anyone on a low fat diet. The method also has a tendency to keep in more of the food's valuable nutrients.

When the clay pot renaissance began in America 10 years ago, all of the models were completely unglazed. This gave the pot a maximum surface for transferring moisture into the pot. After a few years, the manufacturers began to note that some users did not like the slight absorption of strong flavors by the porous clay, and objected to the use of parchment paper to prevent this. Accordingly, they began to glaze the bottom of the inside of their pots.

Eventually they came to offer models that are completely glazed on the inside. But the reason for cooking in clay is the steam effect. The more glaze you put on the pot, the less surface is available for transferring moisture into the pot. It doesn't make sense to buy a clay cooker that is fully glazed inside. The designs with a glaze on the inside bottom are easier to clean and will not absorb flavors into their base.On the other hand, they will not transfer as much moisture. My personal preference is for the totally unglazed models with a bit of parchment paper.

With either model it is important to avoid using the cooker in direct contact with a flame or electric burner, and to keep the pot a few inches away from the sides of the oven. Sudden contact with hot surfaces can cause cracking. The same is true with cold, so put the pot down on a wooden, cork or fire-proof cloth trivet when it comes out of the oven. Don't put it into a refrigerator until it has come to room temperature.

Clay cookers are attractive enough to use as a serving dish at the table, and they keep their heat extremely well. Food left in the pot will stay warm for at least 30 minutes - a built-in hot tray effect. The cooking steam makes the pot "self-basting," yet foods brown well in these cookers. Once the dish is in the oven it takes care of itself.

You can use a clay cooker with any of your standard recipes, just add 100 degrees to the required oven temperature, and 30 minutes to the specified cooking time. The reason for the increase in both cooking times and temperature is that clay will never get as hot as metal.

Some prices: Unglazed Romertopf: 2 to 4 pound, $14; 3 to 5 pound, $18; 4 to 6 pound, $24; to 14 pound, $35.

Glazed Schlemmertopf: 2 to 4 pound, $18; 2 to 6 pound, $23; to 12 pound, $34. CAPTION: Picture, no caption