Thanks for the technical description, Tim, and thanks, Susan, for asking the question. I, too, have seen many whiteware sherds in New Mexico that are "split" through the paste and have wondered how that happened.
Jeff

Jeffrey L. Boyer, RPA
Project Director
Office of Archaeological Studies, Museum of New Mexico
mail: P.O. Box 2087, Santa Fe, New Mexico  87504
physical: 407 Galisteo Street, Suite B-100, Santa Fe, New Mexico  87501
tel: 505.827.6387          fax: 505.827.3904
e-mail: [log in to unmask]

Many a great idea has drowned in the cess pool of data. -Eric Blinman

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-----Original Message-----
From: HISTORICAL ARCHAEOLOGY on behalf of scarlett
Sent: Wed 11/10/2010 7:33 PM
To: [log in to unmask]
Subject: Re: split ceramics
 
Hi all,

All ceramic fabrics are under tension because they shrink as they cool  
after firing, putting stress on the glassy material formed during the  
process.  That is why the ceramic makes a "pling" noise when you flick  
it with your fingernail, like a glass or a metal bell.  The higher  
tone to the pling (the more bell like), the greater the tension.

For highly fired ceramics, any imperfections in the ceramic body will  
cause microfractures which normal "ringing" stress will cause to  
grow.  Cheap white-improved earthenware or CC-ware will sometimes  
crack in this manner, where the stress fractures spread laterally  
within the ceramic and cause it to split into two pieces.  This  
process can be exacerbated by shrinking/swelling as sherds get wet/ 
dry, hot/cold, and so on.  I don't imagine that salt helps either.

I don't think there is anything particularly noteworthy about these  
sherds you have discovered. I have also seen this for creamwares and  
other earthenwares.  It makes calculating MNI or NISP for vessels  
particularly difficult!

Cheers,
Tim

On Nov 10, 2010, at 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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