A CLOSE LOOK AT FLINT PATINA

But first of all, what is flint?

‘Flint’ or what is also called ‘Chert’ is classified as a cryptocrystalline (“hidden crystal”) quartz. This simply means that it is a stone mostly composed of microscopic silica (SiO2) crystals with only minor traces of other elements or minerals. Flint also contains a small percentage of water and is surprisingly permeable when exposed to sub-surface aqueous solutions. This internal water content also helps to explain why flint cracks and pops when overheated and why some artifacts appear to develop instant patina right after they’re exposed to the air and allowed to dry.

Furthermore, archaeological finds world-wide demonstrate that throughout mankind’s history flint has always been a preferred source for the manufacturing of stone tools for three very important reasons: it is hard (7 on the Mohs hardness scale of 10), readily available, and is brittle enough to be flaked in a predictable way that leaves a sharp, durable edge.

Now let’s turn our attention to the actual rock we have to work with in Central Texas: "The Edwards [Flint] Group occur in a variety of nodular and bedded forms, and on the whole, are highly variable in appearance. The heterogeneous nature of this material has led to the generalization of the term "Edwards chert" to represent any gray or tan chert found in West Texas and the Southern Plains (Frederick and Ringstaff 1994; Tunnell 1978; Hoffman et al. 1991) and has encouraged discovery of a simple, expedient means of identification. [But] At this point in time there is no single reliable means of identifying Edwards chert. (Ref 1 p15)

Like other kinds of dense fine-grained quartz, Edwards Plateau Flint [or Chert] is thought to have formed as a silicious ocean bottom ooze supplied by the direct chemical precipitation of aqueous silica from ancient seawater. This reaction in turn cements the micro-skeletal remains of plankton together with tiny fragments of corals, crinoids, snail shells, and other forms of silica rich organic matter into a rigid form.

Now how do we identify and classify patina? Because of its underlying complexity most authorities tend to divide flint patina types into four major categories:

(1) Mineral deposits

(2) White Patina

(3) Gloss Patina

(4) Dark (or Stain) Patina.

Here in Central Texas, mineral deposits usually appear as a rough flaky crust similar to the calcium deposits found in water pipes. Local diggers usually call it “caliche” and some collectors choose to leave it on for authentication purposes while others simply remove it with water and a toothbrush. Its buildup on an artifact is most likely traceable to limestone-impregnated soil layers and likely determined by just how much calcium-carbonate saturated water makes physical contact with the artifact over time.

White Patina first appears as a light blue/white hazing of an artifact’s surface that can progressively become whiter with age. “White patinas, as the name implies, are bluish white, white or light gray in color, often have a splotchy or mottled appearance, and are, in general, are less transparent and generally of a lighter color than the core of the artifact. (VanNest 1985; Schmalz 1960; Hurst and Kelly 1961) (Ref 2)

Bleaching by prolonged exposure to direct sunlight while lying on the surface is often the first explanation that many of us are given, but what self-respecting Native American wouldn’t pick up and reuse a beautifully made tool if it was just out there in the open? Experimental studies would indicate that infrared radiation by itself even in high doses and with a lengthy surface exposure can only produce a slight graying of flint surfaces.

One such recent study Into the Light: The Effect of UV Light on Flint Tool Surfaces, Residues and Adhesives by Michel and Rots concluded: “UV discoloration is moderate even after one year of [full sunlight] exposure… Moreover, the discoloration [whitening] we observe is not comparable (on an experimental scale) to the strong patina often visible on archaeological material, but it [might] represent the beginning of a chain of chemical alterations produced by successive taphonomic* processes leading finally to a heavy patina." (Ref 3) *Taphonomic- [i.e. preservation]

In contrast to light exposure (if it really is much of a factor) most current researchers agree that "the most important process is the dissolution and reprecipitation of silica in the form of a thin film or rind. Patination depends on many factors such as the pH of the environment, soil characteristics (porosity, permeability, soil texture, chemistry, mineralogy …), circulating groundwater, temperature, duration of exposure, the structure of the flint material and even plant root growth." (Burroni et al., 2002; Glauberman and Thorson, 2012; Hurst and Kelly, 1961; Rottländer, 1975a; Schmalz, 1960; Stapert, 1976). (Ref 3 )

Scientifically ‘white patinas’ have been specifically identified and studied since at least 1887 when renowned American archaeologist Neil Merton Judd first suggested that “carbonate-bearing ground-water leached the more soluble amorphous silica, leaving minute interstitial (in between) cavities in the altered margin of the flint. The multitudes of surfaces thus produced reflect and refract light falling upon or passing through the flint, and the resulting scattering causes the white appearance of the altered portion.” (Ref 4 p.1)

The 1994 Fort Hood academic study by Frederick et al. restates this hypothesis as: "White patinas are widely accepted to be the product of selective leaching and result from the scattering of light in the leached areas where there is an increased number of reflective surfaces in the form of grain boundaries and etching pits" (Judd 1887; Curwen 1940; Goodwin 1960; Hurst and Kelly 1961; VanNest 1985; Purdy and Clark 1987). (Ref 1 p.5)