Fresh green or ripe olives are inedible not only because they are hard but also because they are rich in glucosides, a chemical that makes them extremely bitter. While no one knows what prescient mortal in antiquity discovered that leaching softens olives and reduces the bitterness, we do know that they must be cured before they are enjoyed.

There are six basic techniques (each resulting in a characteristic taste and texture) for leaching olives: They can be soaked cracked or whole in water, brine, oil or lye, cured in salt, or sun dried.

This leaching also can be done at home, though it is a long process. However, it gives the cook the opportunity to flavor the olives to personal taste.

The easiest method is water curing as detailed in "The Feast of the Olive" by Maggie Blyth Klein (Aris Books, 1983, $9.95). While the lye method is a bit tricky and usually not recommended for use at home, it can be found in the Time-Life food preserving series, the "Sunset Canning Book" and Klein's book. Send also for the University of California's free pamphlet, "Home Pickling of Olives," by writing to Olives, Branch Center Road, Sacramento, Calif. 95827.

For the interested home cook, some farm stands, supermarkets and specialty stores do sell fresh, unprocessed green (unripe) olives harvested in the fall and early winter, although California olives are frozen and sold "fresh" any time of year. Olives left to ripen and turn dark on the tree are processed or used for oil.

Of course, most olives are leached before they reach the markets. Water-cured olives are soaked in plain water for several months, brine-cured are soaked in salt-water solution, oil-cured in oil and lye-cured in an alkaline solution usually made from lye but sometimes from wood ash. Dry-cured olives are buried in salt and later rubbed with oil. In southern France, olives sometimes are sun-cured, but this method leaves the olives more bitter than the others do.

Once the olives have been cured, whether at home or commercially, they may be packed in brine, vinegar, oil or a seasoned marinade flavored with herbs, spices, onions, hot peppers or orange peel, as in the recipes that follow. WATER-CURED GREEN OLIVES

This is the easiest method of home curing and comes from "The Feast of the Olive" by Maggie Blyth Klein (Aris Books, 1983, $9.95).

5 pounds fresh green olives

1 1/2 quarts water

3 tablespoons non-iodized salt

2 lemons, cut into 1/2-inch cubes

2 tablespoons oregano

2 cups white wine vinegar

6 cloves garlic, peeled and halved

2 tablespoons cumin seeds, crushed

Olive oil

Crack the flesh of the olives by hitting each one with a wooden hammer. Rinse in cold water. Place in a stoneware, earthenware, glass or porcelain jar and cover with cold water. Weight them down with a plastic bag filled with water (to keep olives submerged) and keep them in a dark, cool place for 10 days, changing the water every day.

Boil the 1 1/2 quarts water and dissolve salt in it. Cool. Empty the water from the jar in which the olives have been soaking and rinse olives with cold water. Pour salt brine to cover over the olives in the jar. Add lemons, oregano, vinegar, garlic and cumin. Float enough olive oil on top to cover the surface. Store in a cool, dark place at least 2 weeks. To make a more interesting mixture, add a few store-bought kalamaga olives. Keeps well for at least 2 months. MARINATED CANNED OLIVES

8 ounces California black ripe olives

1/2 teaspoon tarragon

1/2 teaspoon thyme

1 tablespoon coriander seeds, crushed

2 bay leaves, broken

6 large pieces orange peel

1/4 cup white wine vinegar

2 tablespoons salt dissolved in 2 cups water

Olive oil

In a jar just large enough to contain olives and brine, alternate olives with tarragon, thyme, coriander, bay leaves and orange peel so that the spices are well distributed. Add the vinegar; then cover with salt-water brine. Float enough olive oil on top to cover the surface of the liquid. Refrigerate at least 1 week. Serve at room temperature. Use within a month for best flavor. Adapted from "The Feast of the Olive." GARLIC OLIVES

8 ounces pitted California green ripe olives

8 ounces pitted California black ripe olives

1 cup olive oil

1/2 teaspoon salt

4 cloves garlic, crushed

Place olives in a jar just large enough to contain them and their liquids. Add oil, salt and garlic. Cover and refrigerate 24 hours or longer. GREEN OLIVES SEVILLA STYLE

In her new book, "Tapas" (Knopf, 1985, $12.95), Penelope Casas celebrates the tradition of the little appetizers of Spain, served at the many tapas bars that line the big city streets. This marinade, she says, is typically Andalusian and the spices suggest its Arab origin.

7 ounces large Spanish green olives, lightly crushed

1/2 teaspoon cumin

1/2 teaspoon oregano

1/4 teaspoon crushed rosemary

1/2 teaspoon thyme

2 bay leaves

1/2 teaspoon fennel seed

4 cloves garlic, lightly crushed and peeled

4 tablespoons vinegar

4 anchovy fillets (optional)

Place olives in a glass jar in which they just fit. Add cumin, oregano, rosemary, thyme, bay leaves, fennel, garlic, vinegar and anchovies. Fill jar with water and shake well. Marinate at room temperature for 5 days. Refrigerate to store. Serve at room temperature. OLIVE PASTE AND BLUE CHEESE CANAPE (Makes 14 canape's)

Penelope Casas calls this paste "a lovely complement to a robust blue cheese." She suggests using cheese from northern Spain, queso de cabrales, a smooth, pungent goat, cow and sheep milk cheese sold wrapped in leaves, but roquefort and gorgonzola work, too.

1/4 pound cured black olives, pitted

1 large clove garlic, mashed

2 tablespoons pine nuts

3 tablespoons olive oil

14 1/4-inch slices long crusty loaf bread

1/4 to 1/2 pound blue cheese

Place olives, garlic, pine nuts and oil in the bowl of a food processor and chop as finely as possible. When ready to serve, spread thinly on bread slices. Cover with blue cheese and decorate with a piece of olive.