Silver-gray stains on campfire-fired pottery

[Maks]

Hello!

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:

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcS2HIoA2pdj-rtPAralU2qTMPvqgZ3wyLxPQJBhpqD7eS7y3x_s

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.

Best wishes

Maks

 

[glazenerd]

My initial response would be chlorite: greenish grey in mineral and clay form. Also higher in magnesium (grey color) when present with low % of iron and titanium. Chlorite also has mica schist, which may appear as silver, but in reality it is refracting light. Your State Dept. Of Resources should have an area map showing minerals and clays: also available through mine data.

the white in the center is low iron, magnesium, and titanium. The far right is iron (red), and towards the far left is magnesium (grey). From a series of test on oxide levels in natural clay.

tom

[Tyler Miller]

Maks,

Did your firewood have any galvanized nails or fencing staples in it?  Or maybe some scrap metal?  I used to get metallic-grey streaks when I fire raku sometimes because the cans I’d use were zinc lined.

[Maks]

As far as I know - no. The firewood came from whole tree trunks cut into comertially-affordable pieces, without having their bark removed etc. I cut them further with an iron axe, though I would be suprised if any metal particles could came from it. Also the area where the firing was done was virtually free from any possible pollutants of industrial origin - it was done in a Stone Age Open-Air Museum, after all: a sand bottom of a reed-thatched hut.

It's hard for me to make any suppositions, but could birch tar or pine resin compounds present in wood be taken into account? (as far as I remember some pieces were quite resinous). Also, what do You think of clay being overburned, or the fumes fusing with clay bodies?

Most imporant to me is to know aren't those silver stains in any way harmful when in contact with body, but I understand that without a proper anylysis it's rather a play of guesses.

[glazenerd]

Here is a link from  Mindata for chlorite locations in Germany.

http://sciencejournal.in/data/documents/SCIENCE-VOL-1-2-3.pdf. 

The locations are listed at the top of the page. Does the dried clay match any of the sample pictures in color? 

[Maks]

Yes, those with numbers 2 and 3 are very much alike.

As mentioned above, this is the first case of such "staining" I observed - previously after firing the color turned from pale green to "expectable" range of warm yellows, oranges through browns to grayish black - all colors were, however, matte in appearence (still beeing "in" the surface, rather than "over" it). The descibed stains, on the other hand. look like they are applied to it - like brush smudges. The silver gloss is very noticable. Everywhere it appears it is surrounded by a rim of black coloration.

[glazenerd]

This color of silver?

https://en.m.wikipedia.org/wiki/Praseodymium

[Maks]

Yes, the stained surface looks indeed very much like the one on this example:

[glazenerd]

Give me a minute to go find a fired sample. Germany and Poland have large deposits of rare earth oxides.

[glazenerd]

Silver like this? Which is pras. Oxide.

However, magnesium, calcium, and low iron will produce grey. Natural clay blend with clear glaze.

[Magnolia Mud Research]

I am thinking, based on what you outlined in your post regarding the clay, the fuel used, and the firing technique, that the glossy ("silver-gray glaze-like stain") areas are due to the alkalis (potassium and sodium) in the fuel reacting with the clay body.  

The glassy areas are artifacts of a combination of hot spots (=> ~800 C) and active char combustion.  The char combustion releases the alkali from the wood matrix which migrates to the clay surface and on reacting with the surface produces the thin layer of glass on the surface.   The dampness of  wood would help the glaze formation.  This would account for the gloss on an otherwise unburnished clay surface.   Any refractory ash material in the wood (silicon, aluminum, iron,...)already located at the contact point can also be fused (as an oxide) to the surface of the clay by this scenario.

Paul Soldner and others have produce similar glossy surfaces in sagger firings and so-called low fire salt firings. 

I get similar effects by spraying areas on a pot's surface with a dilute soda ash or borax solution.  

The gray color is most likely to due to the soot imbedded into the clay body pores as the glassy potassium rich silicate is clear.   In effect, I think the "silver-gray glaze-like stain" is just a silver-gray glazed area produced by wood firing.   

 
I doubt that the spots are the result of any truly metal buildup.    

I suspect that if you were to wash your fire ashes and keep the wash water to "paint" sections of the pots, and then allow the pot to dry completely before firing, you could force this effect to be occur in every firing.   The ash wash water will be loaded with alkali salts, predominately potassium salts, which will form a glass when reacting with the clay/sand particles in the clay body at spots which reach temperatures above about 500 C. 

 

LT
 

[Tyler Miller]

I feel the need to invoke Occam’s razor here.  The theories are getting a little convoluted.  Salt (NaCl) and soda aren’t likely produce metallic effects.  Blacks, greens, browns, greys, and oranges, are more in its wheelhouse—and that’s when dealing with salt/soda soaked string/cloth/straw wrapped around a piece.

I think the best course of action, if you’re worried about what it is, is to get it tested.   

[Magnolia Mud Research]

I think the "metallic" effect is an optical and mental illusion.   The gloss and reflections from a grey object do resemble a polished metal, but so does a grey object coated with a smooth very thin layer of glossy clear glaze.   My first reaction was NOT metal deposition, but glass formation.  I get the same effect in raku on porcelain sprayed with dilute soda ash or borax, and on the bottom of pots supported on sea shells.   The shiny spot also resembles images from some research conducted in late 1980's to mid 1990's by using clay particles to remove alkali vapor (mostly alkali chlorides and hydroxides) from syngas being fed to gas turbines for power generation.  The "spent" clay particles converted into glazed particles.  The same problems occur now in biomass fueled furnaces.  

For me the "Occam’s razor" answer is that the area is glazed from the alkali in the wood.  all three of the fuel ingredients contain significant potassium species that will if at the clay surface during the early char or pyrolysis stage of combustion will react with the clay and sand components at the surface.  

LT

[Tyler Miller]

Two things prevent what you’re saying from being “Occam’s solution.”

1) I’m pretty sure the potassium salts  present aren’t really reactive in a meaningful way.  My reason for saying this is because I’ve never heard of them being used as colourants.  I’ve also not really heard of a proper vitrified surface from pit fired wares with salt.  In fact, I’ve heard they can effloresce under the right circumstances as the pots age.  Leaving aside the fact that salt doesn’t produce the colours we’re talking about.

If the effect were as you say, we wouldn’t have to stretch for parallel processes like raku (which is still rather hotter) and industrial chem.  In short, it would be a repeatable phenomenon.  and a known one, since pit fires are common. 

2) The established method for getting metallic streaks on ware is fuming galvanized metal.  Eduardo Lazo even recommends galvanized chicken wire as something that can acheive this very effect.

Which is simpler?  An unproven potential practice,  based on analogy?  Or the established practice of x- material produces y result?

Now, the OP has said there was no metal in the pit, so that means my theory is discounted. 

The only way we can know is if it’s tested.

[glazenerd]

https://www.mindat.org/loc-233168.html

The locations of hematite are listed at the top of the page. (If) some hematite is present, that would have some effect on the outcome; hematite is molten at 950C and certainly has the capacity to reduce other minerals. Allophane  is also commonly found in Germany, a clay mineral which often appears green. Figure out the clay, and you can figure out the result. However, like all things clay: the variables are many. A greenish cast in clay often times indicates higher calcium levels: Fullers Earth is a good example ( calcium bentonite)  having a sample analyzed will give the definitive answer.

[Maks]

Thank You for all Your insightful replies. It is fascinating to read through!

Glazenerd:
After seeing the examples You gave, I think the presence of rare metals or hematite is not the case here: as mentioned above, this is the first time I observed something like this, and all objects made of this clay fired previosly were made with material coming literally from the same spot (an eroded bank about 3 m long). If praseodymium had to leave star-like patterns as on the submitted picture than it the anwswer lies someplace else - the stain/marks are patternless, like done with a soft pencil. Also the hematite melting point of 950 takes it out the picture - I doubt a campfire without an external oxygen source could reach a temperature
 above 700-800 Celcius degrees.

Magnolia Mud Research and Tyler Miller:
As said above, the stains or marks look a bit like done with a soft pencil. Given the well-pointed "mental illusion", would You find pausible an explanation with the spots beeing carbon particles  (suspended in smoke) encapsuled under surface molten to glass in clay-wood contact point? Still, isn't quartz's melting point way too higher to allow for sand being molten to glass without, at least, bellows (we speak of a campfire - not a pit; not a kiln, not even an oven of any sort)?
Also, is it possible for clay beeing fired to be spontanously glazed with ash? - like due to a sudden temperature growth (what, as I remember, took place), well after it was covered by it in the first stage of firing. If so, would it be "normal" for such glaze to have graphite-gray color rather than green (like all ash-glazed objects I've seen before - though made only with modern aparature)?

[Tyler Miller]

There’s no way there’s glass formation at that temperature.  There are few minerals that will self glaze at campfire temps, but otherwise it’s not happening.

Also, hematite doesn’t melt at 950 C.  Hematite is Iron (III) oxide, which melts at—it doesn’t.  It decomposes—at a much higher temp.  A fact not insignificant to ceramics.  (oilspot)  It does, however form solid solutions with minerals of similar crystal structure.  Solid solutions can be a tough concept to grasp, but it doesn’t mean the material melts at that point, just that it goes into solution at that point.  I’m in doubt that an ilmenite-hematite solution’s at work here.