| (1) | (2) | (3) | (4) | (5) | (6) | (7) | |
| Minutes | |||||||
| Preparation and building of oven begun | 00 | 00 | 00 | 00 | 00 | 00 | 00 |
| Fire started | 25 | 15 | 06 | 12 | 15 | 15 | 09 |
| More kindling and dung added | 45 | 25 | 25 | 57 | |||
| Oven broken up | 52 | 37 | 36 | 41 | 72 | 40 | 27 |
| Last bowl removed from grate | 58 | 41 | 42 | 47 | 77 | 45 | 30 |
The time required for burning the different wares varies only slightly. Some potters say that red ware takes less fuel, less heat, and less time than polychrome ware; others place both wares in the same oven. The difference in the time required for these two wares is probably no greater than the chance variations in the length of burning as given in Table X. Cooking vessels need to be fired only from one-half to two-thirds as long as the two wares just mentioned (cf. Table X, column 7). The polished black ware will be considered later.
The changes in color before and after firing, and when hot and cool, are very noticeable in some pigments and slips. The orange-red paint is yellow before firing, firing turns it orange-red, almost the color of burnt sienna; cooling produces no noticeable change in this shade. Burning has only a fugitive effect on the red and dark-red slips. While still hot, upon removal from the oven, these are both a dark chocolate shade. As the vessels cool the different reds gradually reappear, until when cold they are the same color as before firing. In some cases, these pigments seem to be just a shade darker after firing than before. The two kinds of white slip are a dead white when applied; after firing they take on a slight pinkish-brown tinge, more cream than white. Cooling produces no further change.
The most interesting change caused by firing is in the black vegetable pigment (guaco). It has already been pointed out (p. 66), that when this paint is laid on thickly, the line made is a little raised and glossy, like a glaze. This “body” to the paint is probably formed by vegetable matter in suspension in the solution. When the vessels with such decorations are removed from the oven after burning, these lines are seen to be white or grey. The material which formed the body of the pigment has been reduced to white ash, which adheres to the surface, but can be rubbed off with the finger or a cloth, while the black color itself has been burned into the clay. Where a thin layer of the pigment was applied, the line, after burning, is light grey, because the black under the thin coat of ash shows through. In those vessels on which a thick solution of pigment was used, the decorations are dead white, and the surface of the ash is often crackled. The thickness of this ash seems to have some effect upon the manner in which the black pigment is burned into the clay, for in decorations covered with a thick layer of ash, the black is of an irregular color with streaks of grey in it. When the thin pigment is used, the resulting black on the finished vessel is of a uniform shade. The glossiness and raised character of the pigment is of course completely destroyed during burning. Guaco lines do not change in shade while the vessel is cooling.
The color of the paste is little affected by burning. In both the red and the white clays, the change is simply one of tone, that is, the clay is lighter after burning than before. The cooking-vessel clay before burning is greyish yellow, but after it has been removed from the oven it is a rich orange-yellow. These cooking vessels eventually become blackened by use over an open fire.
Vessels destined to be polished black ware are treated in quite a different manner during the firing stage, for they are to be subjected to a smothered fire, which will result in the deposition of carbon, thus turning them from red to black. The oven is built in the same way, but greater care is taken to fill gaps, so that more of the heat may be retained. Enough spaces remain, however, to permit free circulation of air. More kindling is used, for a hotter fire is necessary. When the fire has reached the stage at which other wares are removed, it is smothered with new, pulverized, loose manure. Just before the smothering the vessels have the dark chocolate color typical of the red wares while hot. The potter always attempts to smother the entire oven at once by dumping upon it a whole washtubful of fine loose manure. If there is a wind, flames are apt to break out in one or two places. They are, however, hurriedly extinguished. When the loose manure is added, the arch of the oven is, of course, broken, and both cakes and loose manure come in contact with the vessels. For this reason the potter does not exercise particular care in building the oven to keep the cakes from touching the vessels, as they are to be entirely black in the end and a little premature smoking does no harm. After the manure has been added, the mound is continually prodded with a poker to redistribute the loose manure and make certain that all the pieces are equally covered. An extra supply of loose manure is at hand, and this is added in large and small quantities from time to time, as occasion demands. Two washtubsful are generally used, sometimes three. The mound of manure gives off a dense white smoke after the smothering has begun ([pl. 27], a). Great care is taken to prevent flames from appearing, since these would remove the carbon from the vessel.
PLATE 27
a. A fire smothered with loose manure to produce polished black ware. b. Wiping vessels that have become cool enough to handle. One woman wipes off the ashes with a dry rag, then passes them to her companion, who goes over them with a slightly greasy cloth.