Ben and everyone,
All earthenware sherds, including cream-colored ware (cc-ware,
"creamware"), are porous enough that a fragment will easily absorb
water from the broken edges, regardless of scratches in the glaze. As
we've all pointed out, along with the water will come dissolved salts,
minerals, and perhaps metal ions. The fabric will move water through
pores in the ceramic pretty quickly, so long as the fabric isn't non-
porous and totally vitrified (which by definition, isn't possible for
cc-ware).
Most cc-ware ceramics were manufactured by either slip-casting in
plaster molds, thrown and lathed for high end ceramics, then jolly/jig
manufactured after 1840. I don't know if any archaeologists have
studied this, but if you dig into the ceramics engineering literature
from the early 20th century, I bet you will find some clues about how
the microscopic clay crystals orient themselves and how impurities
lead to micro-fractures as a result of these manufacturing processes.
Perhaps the slip cast process causes the clay crystals and other
ingredients to pack more tightly at the surfaces of the mold, while
the interior of the ceramic is of less-densely concentrated material
with a higher percentage of voids. Jig or Jolly work, which combines
the wheel, mold, and lathe into a single machine, may also cause fewer
microscopic voids at the surface of the vessels as it compresses one
side against the rib and the other against the mold. These are just
guesses, however. It is worth noting that cc-ware was made with highly
processed raw material that was milled, sifted, stirred in solution,
and pug-milled. None of these manufacture techniques will produce the
laminated layering one would expect of wedged (kneaded) clay used to
make less refined earthenware.
A more probable explanation is that the glaze causes the fabric's
micro-fractures to expand parallel to the glazed surfaces, which
splits the sherd in half. I would guess this is because the glaze
fills voids at the surface, which discourages cracks from spreading
outward and instead makes them move laterally. The glaze can also
have different expansion and contraction characteristics than the
fabric, which can cause crazing and cracking in the glaze surface when
the vessel swells and contracts through heating and cooling (or
moisture absorption). This could, I suppose, exacerbate the cracking
between the two layers and cause them to separate more quickly.
Another thing to consider is warping. Remember that the whole vessel
is under tension, which is why it plinks. When the object breaks,
some of that tension is released and every fragment warps. Anyone who
has ever tried to glue a porcelain cup back together will tell you
that this can cause measurable changes in shape! So if you consider
that the ceramic body is vitrifiying (becoming glass-like) when it
fires, and shrinks as it looses water volume and then as it cools, it
makes sense that it remains under tension.
Immediately after firing and throughout its use-life, the fabric
develops micro-fissures in the glassy matrix. Then when the vessel
breaks, the sherd warps suddenly. I would guess that the micro-
fissures spread quickly at that moment. The swell/contract cycles
probably move those cracks along until the sherd splits down the
middle like a log on the chopping block.
Pot-lidding is maybe a slightly different issue. One obvious
difference is that the fractures grow through the glaze layer, while
your sherd did not have this happen. On unimproved earthenwares
(pottery) and some improved cc-ware and white-improved earthenwares
(WIE), I have seen "non-plastic inclusions" (i.e. little grains of
rock or mineral, not clay) that cause pot-lidding. This is also
common on brick. If you look closely at little pot-lids on brick, you
will see that many contain a small dot of white or black at the rough
center of the pot-lid scar where the spall has fallen away. This is
caused by the fact that the grain of material, little chips of calcite
for example, absorb water and swell more than the ceramic body. This
puts pressure on the surrounding glassy matrix, like coating a ballon
with epoxy, but then blowing it up after the epoxy has dried and set.
The pressure causes the glassy matrix to split, in a conchoidal
manner, out from the center (where the grain is). The heat of a fire
could cause this, both with a grain of non-plastic material that has
absorbed water or if water has filled a large pore or void in the
ceramic fabric at the time of the fire. As the liquid water changes
phase into a gas, (i.e. becomes vapor,steam), the pressure that
results will cause the same fracture pattern.
So someone could really look into this to satisfy their curiosity. I
bet there is stuff like this written between 1880 and 1930 in ceramic
engineering journals like The Journal of the American Ceramic Society
and The Journal of Industrial and Engineering Chemistry. As I said
before, I don't think that this research will really improve your
understanding of the site's occupants or their lives. It would be an
interesting study from the standpoint of understanding the behavior of
cc-ware manufacture and decay, which are very, very understudied
archaeological topics! Could be a great experimental archaeology
project!
I must get back to real work.
Cheers,
Tim
On Nov 12, 2010, at 12:40 PM, Benjamin Carter wrote:
> All,
>
> I love the idea of using the fracture patterns from frost/rain/salt/
> other split pottery to get more information about the formation of
> the paste or matrix of a ceramic. I would think that this would be
> fairly complicated. It would seem that breaks for hand built coiled
> pottery would be fairly obvious, but what can we see from industrial
> products such as creamware? I'd love to hear what you all have to
> say. As I mentioned before I have a large number of very tiny
> fragments which don't tell me much more than the basics. Any way to
> get more information would be wonderful.
>
> Cheers,
> Ben
>
> On 11/12/2010 12:12 PM, David Legare wrote:
>> See Mark's comment below. The addition of water to the paste from
>> the processes
>> he mentions will also cause that kind of spalling or "potlidding"
>> in the
>> ceramics. It is the same thing that happens to cherts and
>> chalcedonies with
>> enough heat. I suspect that scratches in the glaze or unglazed
>> portions of the
>> material will allow that kind of water seepage.
>>
>>
>>
>> ----- Original Message ----
>> From: Conrad Bladey<[log in to unmask]>
>> To: [log in to unmask]
>> Sent: Fri, November 12, 2010 9:55:48 AM
>> Subject: Re: split ceramics
>>
>> the differential effect of freeze thaw upon "pastes" or clay bodies
>> may
>> be one way of separating what would be otherwise plain
>> undistinguishable
>> wares.
>> Given similar conditions some would and some would not. However this
>> could have to do with the way the particular batch of mix was
>> blended.
>> Like bread dough when kneeding striations develope as in muscle.
>> Between
>> the strands the water gets in then freezes then pops.. With more
>> mixing
>> some wares may resist even though they look the same. They may not
>> come
>> off in layers but in other ways.....never resist an opportunity to
>> "split"
>>
>> Susan Walter wrote:
>>
>>> Yes, I have encountered those too; but not at this site. These are
>>> split right down through the paste, with each side piece one side
>>> glazed the other "raw."
>>>
>>> This site also has some of the typically melted and discolored
>>> surface
>>> glaze from the burn.
>>>
>>> Anyway, what satisfies me is every one responding says it is a
>>> natural, and not really uncommon phenomenon.
>>>
>>> Thanks everyone!
>>>
>>>
>>> ----- Original Message ----- From: "Patrice L. Jeppson"
>>> <[log in to unmask]>
>>> To:<[log in to unmask]>
>>> Sent: Thursday, November 11, 2010 6:26 PM
>>> Subject: Re: split ceramics
>>>
>>>
>>>> I have seen ceramic fragments at the site of an early 19th C
>>>> house fire
>>>> that exhibited substantial spalling. The fragments looked
>>>> pockmarked.
>>>>
>>>> On 11/11/2010 10:49 AM, Susan Walter wrote:
>>>>
>>>>> Not at all near salt water; inland a 2 hour drive from the
>>>>> ocean. And
>>>>> the locus is on a low hillside with an intermittant stream a few
>>>>> hundreds of feet away.
>>>>>
>>>>> One thing I did not mention was this particular site is supposed
>>>>> to
>>>>> have existed less than 3 years, and was burned out. And there
>>>>> have
>>>>> been wildfires through the area for hundreds of years, though it
>>>>> was
>>>>> spared that during the last few years of our infernos.
>>>>>
>>>>> Has anyone seen this splitting/spalling effect as a result of
>>>>> fire?
>>>>>
>>>>> ----- Original Message ----- From: "Benjamin Carter"
>>>>> <[log in to unmask]>
>>>>> To:<[log in to unmask]>
>>>>> Sent: Wednesday, November 10, 2010 4:29 PM
>>>>> Subject: Re: split ceramics
>>>>>
>>>>>
>>>>>> Susan and other Histarchers,
>>>>>>
>>>>>> Could it be salt water getting into the sherd? When salt water
>>>>>> dries,
>>>>>> the formation of crystals can cause ceramics to spall/split.
>>>>>>
>>>>>> Nearly all of my white earthen ware ceramics from a coastal
>>>>>> Maine site
>>>>>> (c. 1800) were split. I think it was due to a combination of
>>>>>> freeze/thaw
>>>>>> cycles and salt water. At this point most are from relatively
>>>>>> close to
>>>>>> the surface.
>>>>>>
>>>>>> Ben Carter
>>>>>>
>>>>>> On 11/10/2010 6:18 PM, Susan Walter wrote:
>>>>>>
>>>>>>> Hmmm, the site is in San Diego County...
>>>>>>>
>>>>>>> ----- Original Message ----- From: "Mark Branstner"
>>>>>>> <[log in to unmask]>
>>>>>>> To:<[log in to unmask]>
>>>>>>> Sent: Wednesday, November 10, 2010 11:40 AM
>>>>>>> Subject: Re: split ceramics
>>>>>>>
>>>>>>>
>>>>>>>> Susan,
>>>>>>>>
>>>>>>>> I don't know what part of the country that you're working in,
>>>>>>>> but
>>>>>>>> "frost spalling" is a relatively common phenomenon ...
>>>>>>>>
>>>>>>>> Essentially, moisture gets into the porous fabric of the
>>>>>>>> pottery and
>>>>>>>> it splits the pottery when it freezes during the winter. It
>>>>>>>> certainly does not happen all the time, but it does occur
>>>>>>>> with some
>>>>>>>> regularity. Sometimes just the glaze spalls off the fabric,
>>>>>>>> but the
>>>>>>>> fabric itself can split.
>>>>>>>>
>>>>>>>> Mark
>>>>>>>>
>>>>>>>>
>>>>>>>>> Nov. 10, 2010
>>>>>>>>>
>>>>>>>>> Hello all,
>>>>>>>>>
>>>>>>>>> I have a site with several sherds of transferware that are
>>>>>>>>> split so
>>>>>>>>> that there is a transferware / glazed surface on one side
>>>>>>>>> and the
>>>>>>>>> paste interior on the other side.
>>>>>>>>>
>>>>>>>>> This makes them about 1/8 inch thick. They tend to be about
>>>>>>>>> 1 inch
>>>>>>>>> long, and about 3/4 inch wide.
>>>>>>>>>
>>>>>>>>> The edges are not shaped; in "plan" view the pieces are
>>>>>>>>> miscellaneously trapezoidal.
>>>>>>>>>
>>>>>>>>> In appearance they somewhat remind me of porcelain pin
>>>>>>>>> trays/childrens toy tea trays that are decorated and glazed
>>>>>>>>> on the
>>>>>>>>> top and bisque on the bottom, except these transfer pieces
>>>>>>>>> are not
>>>>>>>>> smoothly finished on the bottom ("bisque") like the
>>>>>>>>> porcelain items
>>>>>>>>> are.
>>>>>>>>>
>>>>>>>>> I do not think these were made intentionally.
>>>>>>>>>
>>>>>>>>> I've not seen so many of these at one site before. Has anyone
>>>>>>>>> knowledge of what did this?
>>>>>>>>>
>>>>>>>>> Thanks every one,
>>>>>>>>>
>>>>>>>>> S. Walter
>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>>
>>>>>>>> Mark C. Branstner, RPA
>>>>>>>> Historic Archaeologist
>>>>>>>>
>>>>>>>> Illinois State Archaeological Survey
>>>>>>>> Institute of Natural Resource Sustainability
>>>>>>>> University of Illinois at Urbana-Champaign
>>>>>>>> 209 Nuclear Physics Lab, MC-571
>>>>>>>> 23 East Stadium Drive
>>>>>>>> Champaign, IL 61820
>>>>>>>>
>>>>>>>> Phone: 217.244.0892
>>>>>>>> Fax: 217.244.7458
>>>>>>>> Cell: 517.927.4556
>>>>>>>> [log in to unmask]
>>>>>>>>
>>>>>>>>
>>>>>>>> When you think of that perfect retort five minutes after the
>>>>>>>> conversation is over - that's when you're:
>>>>>>>>
>>>>>>>> "... bursting with the belated eloquence of the
>>>>>>>> inarticulate ..."
>>>>>>>>
>>>>>>>> Edith Wharton in the "The Age of Innocence"
>>>>>>>
>>>>>>>
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>>>>>>> 11/09/10 23:34:00
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