Small Scale Thermal Alteration

Thermal alteration

            Scott Jones, in his book A View to the Past, has done an admirable job of explaining the scientific reasoning behind the process of thermal alteration, however, even Scott admits that the Indians did not need and probably did not have a deep understanding of this process in order to use it effectively in their tool development. All they knew was that it worked.

In brief, the process of repeatedly heating, cooling, and reheating some types of rock, coastal plains chert in this instance, to temperatures of 250 degrees or so with a maximum temperature of 300 degrees Fahrenheit, would cause the minutely crystalline structure of the rock to be “reorganized” into a yet finer structure, thus making the stone more workable, harder, and cause it to hold a finer edge. Jones also discovered in his studies that the further away from the quarry site the stone traveled, the more likely it was to be altered and used as a more lasting tool. It seems that even the higher quality chert was routinely altered to further improve its quality.

Jones seems to have focused his experimental practice on the methods seen in Late Archaic camp sites while he readily admitted that other methods he discussed may have been used, but lacked archaeological evidence. The process of thermal alteration as seen from the projectile points and blades produced began as early as the Transitional Paleo to Early Archaic period. I have seen only two thermally altered Paleo points and those may have been altered later in time by various surface fires with natural causes. The entire practice may in fact have been learned by early man by recovering rock resources that had been associated with earlier fires with natural causes.

            The following steps have been adapted from A View to the Past by Scott Jones (page 157-158) using a camp fire as a means of achieving thermal alteration on a small scale for personal use. Large scale alteration using a kiln or oven is also possible, but should be done under the supervision of a skilled craftsman. Even in the campfire setting, care must be taken not to expose the rock to open flame or to excessive heat as chert contains small amounts of moisture and can explode when overheated.

  1. Build a small, dinner-plate sized platform of rocks. Use thermally stable rocks such as sandstone, granite, quartz, or other thermally stable rocks. Your platform may be above ground with a ring of larger stone to contain the required sand, or it might also be placed in a shallow impression in the ground for the same purpose.
  2. Fill in the spaces between the rocks with dry sand.
  3. The small cores, bifaces preforms, or spalls of stone to be altered should be spread over the sand-filled platform. Thin blanks or flakes are best as they handle the stress of alteration better than larger, thicker pieces.
  4. Cover the platform and stone preforms or spalls with an additional inch of dry sand.
  5. Build a fire fueled with small twigs and sticks over your hearth.
  6. Build your fire up slowly, allowing the twigs to heat the hearth up before adding small stick of no more than 1.5 inches in diameter being careful not to disturb the sand over the hearth.
  7. Maintain your campfire for several hours. Having an alternate use for the fire such as cooking will keep you attentive to your fire over the required time.
  8. After several hours of heating, the stone’s structure will be sufficiently altered to allow it to be worked after cooling. Allow the fire to cool slowly and retrieve your spalls the following day.
  9. While some of the spalls may be sufficiently altered, others may need to be reheated to further alter them for use.
  10. When altered, the trace minerals present in the stone will have caused the stone to turn various shades of a wide range of colors. The stone will have also taken on a waxy feel and may have a slight gloss to the surface.