This section is from the book "The Gardener's Monthly And Horticulturist V18", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
A system prevails, more or less, in all wine-producing countries, but especially in Spain, of what is known as " Plastering " the Wines. In some notes on the chemistry of tartaric and citric acid, in a recent number of the Journal of the Chemical Society, Mr. B. Warington gives some interesting particulars relating to this plastering. It consists of treading in with the grapes a kind of plaster known in Spain as "yeso," about 10 lb. being added to the quantity of grapes required to make a butt of wine (108 gallons). The " yeso" reduces the acidity, and "is used equally for red and white wines, but is not employed in the preparation of the sweet wine 'vino dulce,' made from over-ripe grapes for the purpose of tempering sherry." Besides plaster other substances are sometimes added to wine, and affect the composition of the lees. A substance known as Spanish earth is commonly employed in Spain for fining the wines. This earth is also occasionally used by English wine merchants for a similar purpose.
This so-called Spanish earth has a somewhat soapy feel, easily impressed by the finger-nail, but containing sometimes fragments of slates. "When rubbed in the hands under water the earth is resolved into an mictions paste, and it is in this condition applied to the wine which it is destined to clarify."