Hunter-Gatherer Archaeology of the Colorado High Country

Appendix J. Experimental Firepit Construction

We have conducted preliminary experiments to evaluate how firepit construction influences burning characteristics. Our experiments entailed construction of firepits that duplicated some of the prehistoric variation observed. We built a slab-lined firepit, an unlined firepit, a shallow depression firepit, and a shallow cobble-filled firepit. We repeated combustion of equal weights of conifer and sagebrush fuel in these firepits. A recording thermocouple measured the heat generated from the firepits every thirty seconds. Our slab-lined firepit was fifty centimeters in diameter and forty centimeters deep. It was lined with sandstone taken from exposures at the Tenderfoot Site. Three times we burned 14 kg of conifer-wood scrap lumber. The rock became fire-reddened but did not shatter, except for some minor spalling along bedding planes. The walls of the firepit stayed intact and did not slump. The flame lasted about three and one-half hours. Our unlined firepit was fifty centimeters in diameter and forty centimeters deep. The walls were vertical before the first firing. Subsequent burnings were conductedinthesamepit;thefire-reddenedwallsbecameveryfriableandslumped, and the pit got larger in diameter. With pieces of wood longer than the pit, one end was put into the pit and the other end was rested on the lip of the pit. Where the wood sat inside the pit, undercutting occurred, and where the wood rested on the edge, the lip of the pit became smoke blackened and rounded. Flames were present for about two and one-half hours. The sagebrush fires were built in a depression fifty centimeters in diameter scooped out of the surface to a depth of ten to fifteen centimeters. Six kg of sagebrush served as the fuel for each burning. The first firing was done with no rocks in the bottom of the depression. The next two were done after the depression had been lined with a single layer of fist-sized cobbles. In all cases, flames were absent after about an hour. Appendix J Experimental Firepit Construction 278 Appendix J We burned 14 kg of conifer wood three times in the slab-lined pit and three times we burned the same in the unlined pit. We burned 6 kg of sagebrush fuel in the shallow depression, once without rocks and twice with rocks. The thermocouple measurements are shown in Figure J.1. The first measurement of each firepit was done with a short-wire thermocouple, which came out of the slowly collapsing charcoal and ash. These events are shown on Figure J.1 as points on the burn line at which the temperature quickly declines. Although this method precluded our obtaining some information about the fire retained by firepits, it did produce information about the heat given off by them. In the later experiments, we used longer thermocouples and the sensor stayed in the coals longer. As is shown in Figure J.1, the unlined firepit provided a very hot fire that lasted a relatively short time. Although the slab-lined firepit did not get as hot as theunlinedpitdid,theheatlastedsignificantlylonger.Theslab-linedhearthfilled with coals during burning. The fire burned more slowly and it was easier to control when using the slow-feed method. The sagebrush-fired pits cannot be directly compared to the conifer-fired pits because different amounts of fuel were burned. However, it can be said that sagebrush produces a remarkably long-lasting fire. The sagebrush fires burned Fig. J.1. Thermocouple measurements for firepit experiments. Appendix J 279 Table J.1—Results of flotation separation of experimental firepit contents. Weight in Grams Sage Fire Rock-Lined Firepit Unlined Firepit Total flotation sample 820 940 7,558 Light (charcoal) 39 484 275 Heavy (soil) 258 240 4,340 Residual (silt) 410 213 2,700 Total (recovered) 707 937 7,315 rapidly as each new piece of wood was placed on the fire and after the initial flareup settled to a steady burn. After all the fuel was added and the flames died down, the sagebrush-fired hearth gave off heat that, while not detectable very far from the hearth, did last a long time. The “fill” of each firepit was removed after each burning. One time for each type of firepit—slab-lined, unlined, and shallow rock-filled (sagebrush-fueled)— the contents were retained and processed through flotation analysis. The results of the flotation analysis can be seen in Table J.1. The slab-lined firepit yielded more charcoal than either of the other firepits. The...