I've been working with clay for few years, trying to rely on local (self-gathered) resources as much as I could. Beeing interested in prehistoric (Neolithic and Proto-Neolithic) pottery I also struggle to complete the products in a period-correct way, firing them in an open or semi-open fire, rather than a modern electric kiln.
Co-working with one Stone Age Open-Air Museum in Germany, recently I went there to fire some of small pieces made of my local clay: vessel flutes (ocarinas) inspired by the archeological finds and some small pendants, also based on the excavations from different prehistoric European cultures. After firing some of the finished pieces happened to be stained with a silver-gray, metal-like spots on an uncertain origin. Though that wasn't the first firing done by me in such "primitive" way, such thing I observed for the first time. After asking two professional potters: an eldery prehistoric pottery expert working in the Open-Air Museum and a friend of mine, a modern professional ceramic artist, I recieved no concrete answear - by the latter it was suggested that this may come from overburning, what I - comparing the case with a set of images while keeping in mind short firing time and low firing temperature - had to doubt.
The clay used comes from an eroded strem-bank in a beech-dominated forest: gray-green when wet, turning to pale green when dry. Taken out of the deposit it naturally contains a lot of pebbles varying in size from 0,5 to about 4 mm. The fired pieces were made without temper, with most stone pieces removed prior to forming. Dried for about a month they were fired in a campfire, built in the following fashion: a "well" of thick wood pieces, with pottery lain on a single wood layer in the bottom, covered to the top with a mass of wood chips, smaller wood and bark pieces. The fire was burnt from the top, letting the clay below to absorb temperature, while keeping the "well" structure stable.
Given very small scale all objects went well through the firing, exposing normal varied yellowish and blackish staining from reductive atmosphere, as most prehistoric and early historical pottery does. On the surface of both the vessle flutes and one pendant there was, however, this mentioned "metallic" staining, on one flute appearing around a point where a very small piece broke off, exposing a shiny piece of some (?) material within the ceramic mass. It is also present on different spots, surrounded by black reductive "rim".
The wood species used were oak, birch and pine, with oak beeing the most abudant, pine the least. Some of the wood pieces, beeing kept outside, were a bit damp..
I know of a distinct type of pottery called "siwaki" (shee-vah-kee), or "the grays", used formerly in some regions of Poland, having this name given thanks to a process of "smoking" the almost-fired pots with fumes of wood being put in the kiln prior to the end of firing, "sinking in" the surface, resulting in (probably similiar as in my case) silver-gray glaze-like stain. The gloss is said to appear thanks to a careful polishing, which, in the described case, wasn't done - the surface was left rough, with a set of hand-marks.
A picture of multiple siwak pots, for comparison:
I send a picture done after firing. The silver-gray staining is visible as a spot over a hole in the vessel flute on left, also with a small chip of clay broken off, exposing a shiny piece within.
Since the object are to be usable and kept next to the body (as musical instruments and ornaments), I want to be very certain that the staining doesn't come from any potentially harmful substance (like for example lead - if its admixture would be by any chance possible).
I would be more than happy if some of You, informed in clay chemiastry, could clarify, how could those stains appear and what kind of processes are behind them.