Overview

 

This page is for students working on the Pottery Making video podcast. It will have the following features:

• Student notes from their individual research, edited into a draft script 
• Revisions by team members and the Subject Matter Expert
• Questions to ask the SME and others during the interviews at the sites
• Transcripts of video clips of the interviews and tours
• Final approval by the SME and company PR people 

 

Team

 

• Debbie Gines: History of pottery, types of pottery and kilns 
• Kayden Valentine: Chemistry and environmental impact of pottery 
• Marc Teeples: Processes of pottery making
• Barbara Rowland: Kilns, Colors and glazing of pottery

 

Timeline

 

• Initial Research: November, 2008 
• Detailed Research/Draft of Script: Dec. 15-16, 2008 
• Contact with SME: By Dec. 19, 2009
• Tour of Companies (Videotaping): Jan. 20-23 , 2009
• Capture and Transcription/B-Roll Creation: Jan. 26-30, 2009 
• Final Script Complete with Approval of SME: Feb. 6, 2009 
• Video Editing - Draft: Feb. 9-13, 2009 
• Alpha Testing/Revisions: Feb. 17-20, 2009
• Beta Test/SME Checkoff: Feb.23-Mar. 2, 2009
• Export to Podcast/Add Metadata: Mar. 5-9
• Post to Blog, Upload to iTunes: Monday, Mar. 12 

 

 

Script Sections:

 

Lead In:

 

 

History of Pottery: 

 

   Pottery is one of the oldest and most useful of the industrial arts. 

As most things are invented out of a need, so was pottery.  Many cultures were able to use hollowed out stones, or wood for their needs, but these were not good for traveling.  Almost every culture was able to construct baskets using willows and branches etc.. but these were not made to hold liquid.  Thus the idea of forming clay came to be.  The history of pottery can be dated back to 6000 BC at the begnining of the  period in the Middle East.

The process of making a simple bowl is:

Gathering clay from the earth and making it into a ball (the size of the ball depends on how big you want your bowl to be).  Then a hole is pushed into the ball of clay creating a "bowl".   With this method the clay was usually not "fired" but used only once and then discarded.  Another method used was to make a long snake out of the clay and coil it into a pot shape.  This was then fired over an open camp fire, making them ceramic, then saved to be used again.

 

   The potter's wheel came into the picture around 3000 BC during the Bronze Age.  The wheel was in a plate form made of wood that the pot was built on top of.  It was able to turn so that the potter could sit instead of walking around and around which made the process much faster.  By 2000 BC the wheel was invented in Europe and Asia. This was also a plate like platform but it was a bit different in that it could spin like a top on an axis.  This way they could start spinning it and it would keep spinning on it's own.  A good potter could usualy make one bowl per min. using this method which enabled them to sell their products much cheaper. 

Later, the Indo-Europeans migrated into Greece, Italy and China bringing this idea with them.

 

   Pottery is very different depending on which part of the world you are in.  People used pottery as their social identity.

 

At the beginning of the Roman Empire, technological and economic changes made huge effects on how people made pottery.  For instance, they started painting them red and them eventually replaced the usual painting methods with using molds.  At the same time the invention of glass blowing by the Phoenicians became a serious competitor with pottery.

 

 

 

 

   Around 700 AD in China, they began to make porcelain (a gleaming white) potteryand perfected the art of porcelain pottery.  They also began using colored glazes to put designs on.

 

 

 

Process of Pottery Making:

 

• Clay preparation- This consists of either: Finding, making, or digging your own clay.
• Wedging- Process of mixing the clay by hand by kneading a clay ball on a table.
• Forming/Throwing- After you wedge the clay you can form it by a slab, wheel, coil, pinch, or mold.
• Drying- You let it sit out to dry so all of the moisture leaves the work, and it becomes stiffer and shrinks. You want to wrap your work in plastic and keep in a damp room.
• Leather Hard- After it’s in the room a few days it turns to “leather hard,” which is when some water has evaporated from the piece and it’s partially dry.  This is the last chance to make any final shapings, carvings, or to add anything onto the piece.
• Greenware- This is when your piece has dried completely. All of the water has evaporated from it and it is very brittle. You then put it on the greenware rack where it waits to be put in the kiln.
• Bisque Firing- The firing process is a 3 day process. The first day it is put in just to be slowly warmed up over night. Putting the temperature too high can cause the piece to explode. Next you slowly turn up the temperature between day 2 and 3 to the max of 1,800 degrees farenheight. You then take it out to set on Bisqueware. This process makes it a lot less brittle, and easier to handle.  
• Glazing- The glaze is made out of ground glass, clays, coloring materials, and water. You then cover the  piece by dipping, pouring, spraying, brushing, sponging, or a combination of them.
• Glaze Firing- It reaches a 2,500 degree temperature. Another 3 day process and after 3 days you leave to cool.
• Overglaze Firing- Extra glaze or glaze touch ups, fired at a very low temperature of 1,300 degrees.

 

Types of Pots:

 

   The three major types of pottery are earthenware (terra cotta pots for plants), stoneware (common dinnerware), and porcelain (bone china dinnerware, figurines, vases).  The type of clay and the temperature it is fired at determines which group pottery falls into.  Earthenware is more prone to cracking and chipping than the other two.  However the lower firing temperature of earthenware permits a wider range of glazes.  The clay mixture used for stoneware is the heaviest of the three, and therefore imparts more strength to the pottery.  The higher firing temperature of stoneware furthers adds to the strength.  Porcelain, commonly called china in deference to its place of origin,  is the most delicate of the three.  There are three types of porcelain.  Porcelain  is fired at a low temperature.  http://42explore.com/pottery.htm

 

 Kilns: 

 

   As a wandering hunter-gatherer, a person was not able to own more than he could carry.  An important side development of becoming an agricultural society, which not only created the need to have more permanent storage containers, was the ability of  just being able to have them.  Thus, the invention of the kiln contributed to the phenomenon known today as “keeping up with the joneses”.

 

   Turning “mud” into a permanent item requires heat.  The earliest kilns, known as pit kilns, were simply trenches dug in the ground, filled with the clay articles, burning wood, and then covered over with dirt.  Today kilns are highly insulated, energy efficient, computerized chambers allowing the potter to control the temperature rise (ramp), to maintain the desired temperature (soak), and to control the rate of cool down.

 

The transition from Neolithic trench to modern technology had a few improvements along the way.  The use of dampers, fire boxes, and chimneys were early improvements.   The Romans, throughout their empire,  not only made use of these developments,-  they enhanced them by building their kilns on slopes, positioning the fire box at the bottom to provide indirect heat.  http://en.wikipedia.org/wiki/Kiln

 

 

  

Roman Kiln (missing roof)

Castor, England

The Encyclopaedia Britannica

 

 

 

   Around 500 AD, the Chinese took the technology of the slope kiln and modified into the climbing downdraft kiln.    The up and down movement of hot air within the climbing kiln made for more effecient use of  the fuel.  Clay sagger boxes within the kiln kept fuel ash off the pottery and glaze.  These kilns were so large, they were used by entire villages for mass production of pottery.  Because of their size, climbing kilns were not easily hidden, and as a result their technology eventually migrated throughout Asia and Europe.  It was within climbing kilns that the Chinese perfected their skills in porcelain, safeguarding the secret of porcelain for over 700 years.  http://seco.glendale.edu/ceramics/kilns.html

 

   The electric kilns of the of today have little in common with their fuel burning predecessors.  Other than an enclosed area for holding the pottery, the only other thing they share is the need for insulation.  Since electric coils provide the heat necessary, there is no need for a fire box (fuel), damper (oxygen control), or chimney (exhaust).  Since it doesn’t require feeding the fuel box, no constant supervision is needed, allowing the whole process to be controlled via a computer. 

 

  

  

Olympic Electric Kiln   

Model 2327HE/240-208v

www.greatkilns.com

 

 

 

Coloring and Glazing Pots:

 

   Pottery can be finished either by painting or by glazing.  Paints are applied to bisqueware (pottery fired once) and not refired.   Glazing is a medium that is applied and the pottery is then re-fired.  The painted finish is more of a matte, as opposed to the glass finish of glazing.

 

   People have probably been glazing pottery for almost as long as they have been making pottery.   Glazing serves two purposes: it makes earthenware pottery waterproof (stoneware and porcelain become waterproof upon the first firing), and it adds visual interest.  All glazing is added after the first firing, regardless of type of pottery, and is added to the interior and/or exterior of the piece.   The bottom of the piece is either not completely glazed or the piece is put on a kiln stilt (stand) to prevent the glaze from adhering to the shelf in the kiln, ruining the pottery piece and also possibly the kiln shelf.

 

   The types of glazes must be compatible not only with each other, but also with the type of clay upon which they are used.  Generally one layer of glaze is added per one firing.  Specialty glazes may require more than one firing.  Gold, silver, and mother of pearl type glazes are the last layer of  glaze added.   However, the more a piece is fired, the weaker the clay becomes.  Pottery used in junction with food and beverages must be glazed to seal the surfaces.  The glazes used must be lead free.  Glazes are clay type specific.  Glazes  used on stoneware require different firing temperatures than those used on earthenware and porcelain.   Glazes for one type  of pottery should not be combined with glazes for another type on one piece.  http://www.lockettpots.co.uk/pot_glt.php

 

   While glazing of pottery was widely done, various cultures around the world used different types of glazes.  English potters used a salt glaze,  Near Eastern potters used alkaline glazes.  Probably the best known potters /glaziers were the Chinese, who considered glazing of high importance in the finished work, and used various types of glazes and techniques.  http://www.artistictileandstone.com/pages/Info/Info_pottery.html

 

   Glazes are made from a vast array of minerals, and other materials such as inks, carbons, and resins.   http://www.thomasglobal.co/about%2FENUS%2F132940.htm   They are liquid or powder mixed with water to make a liquid and then brushed on the pottery.  Glazes may contain solid particles that melt during the firing process that add to the final finish.  The mineral alumina controls the flow rate of the other minerals that have become liquid due to the intense heat.  The cooling of these liquid minerals result in the glass-like finish of the glaze. 

http://ceramics.sdsu.edu/articles/glaze_tech/simple_glaze_chemistry.html

 

  The chemistry of glazes is just that, chemistry.  While pre-made glazes can be bought,  the adventurist may prefer to make his own.  Knowing what mineral to use, in what amount, and how it reacts in combination with the other elements is what results in successful glaze formulation. 

 

   While the basic recipe for making glaze contains five ingredients:  flux, stabilizer, opacifier, colorant, and glass former;  the ingredients can be combined in varying amounts to give an almost infinite  number of glazes.  And like a food recipe, the specific ingredients, along with their measured amount, need to be written down so the resulting glaze can be  mixed again if so desired.  With silica as the major oxide used for the glass forming element and alumina as the most used stablizer, deciding which flux and the amount to use is the crux of glaze formulation.

 

   Flux is any ingredient that assists in melting  the glaze or  in lowering the melting temperature of the glaze.  Common fluxes are limestone, potash, and talc.  A stabilizer is any ingredient  that extends the melting range of a glaze.   They also stiffen the glaze.  Alumina is the major stabilizer, with china clay the second most used stablizer.  Glass former is the main ingredient of any  glaze and requires  additional melting above that required for  the fluxes and and stabilizers.  Silica is the main glass former.  The amount of silcia used determines whether a glaze is dull (less silica )or shiny (more Silica).  Opacifiers make the glaze more opaque by hindering the passage of light through the glaze.  Tin and zirconia are common opacifers.  Colorants color the glaze.  They are usually metals and only a small amount is required.  Some colorants are copper, iron,  cobalt, and nickel. 

http://www.duncanshearer.co.nz/glaze/glazechemistry.html 

 

   In their unfired state, glazes have no resemblance to their fired finish.  The color of unfired glazes gives no clue to what the fired color will be.  The texture of unfired glaze is that of  dried powder, with the finish of most fired glazes being smooth colored glass.

 

 

Glazing Defects:

 

  A common defect in glazes is pitting, the small pinholes in the glaze resulting from dust on the biqueware or gases escaping the glaze during firing.  A dust-free piece and firing at the correct, usually higher, temperature will lessen pitting.

 

   One of the most common glaze defects is crazing. Crazing is unwanted hairline cracks in the finished glaze.  It is caused by the glaze shrinking smaller than the pottery during the cool down period.  It may be evident immediately or develop over time. While crazing is generally an unwanted development, it is on occassion the effect wanted, in which case it is called a crackle glaze.

 

   Shivering is opposite of crazing with the pottery shrinking more than the glaze, lifting the glaze off the surface of the pottery.  Shivering is the undesired result of improper glaze formulation.

 

   Crawling is crazing gone wild.  Rather than being hairline cracks in the glaze, the glaze separates into individual islands of glaze with the unfinished pottery showing through.  Crawling can be caused by the glaze not flowing properly during the firing, by dusty bisqueware, or the wrong firing temperature.    http://www.pottery-magic.com/pottery/glazes/glaze-defects.htm

 

 

 

 

 

 

   

Typical Wedgewood blue and white cameo             typical Delftware panel

 photo by Lionell Allorge                                        tin glazed earthenware                                           

 

 

 

 

 

 

 

Chemistry of Pottery: 

 

In the chemistry of pottery making, the clay undergoes two basic stages in the firing process, Dehydration and vitrification.

 

     The Dehydration process is essentially complete at about 500 degrees Celsius. Beyond this point it begins to become apparent in the clay that sintering, or the becoming of a coherent mass by heating without melting, begins.  The molecules of silica and alumina begin to collapse together; partially filling the empty pockets left by the escaped water and creating a relatively firm bond which gives the fired clays body its hardness and strength.  This is the earliest stage of vitrification.  At this point the pottery will no longer absorb water chemically.

 

     The Vitrification process is the continuance of the melting of some of the impurities in the clay, normally starting with the iron oxides. They combine and melt forming a glassy substance that flows around the remaining alumina molecules gluing them together to make a rigid bond. Then at higher temperatures mullite and alumina silicate begin to grow like need-like crystals and become interwoven with the glassy substances in the clay body.  This then leads to another smaller process known as Glassification where the growth of mullite combines to form an extremely rigid and hard crystalline structure in the clay body.  The Glassification process gives stoneware and porcelain is characteristic of the clear ring when tapped.

 

Environmental Impact of Pottery:

 

The process of creating pottery is the main factor of pottery’s environmental impact. In the firing process potential pollutants are released into the air such as carbon dioxide, sulphur dioxide, fluorine, and possible chromium.   Most of these gases build up into harmful greenhouse gases which add to more greenhouse gases from other man made products which effect the atmosphere and climate change.  The increasing concentration of gases leads to trapping of more heat in the atmosphere which leads to higher temperatures.  These gases continue to increase the rate of change in the atmosphere which is changing faster than what the normal planet’s organisms can adjust to.  All of these changes in the atmosphere can lead to reduced rainfall, rising sea level, and extinction of plants and animals that cannot adjust at the faster rate.

 

 

Summary:

 

 

Credits:

 

http://www.lockettpots.co.uk/pot_glt.php

http://www.artistictileandstone.com/pages/Info/Info_pottery.html

http://www.thomasglobal.com/about%2FENUS%2F132940.htm

http://ceramics.sdsu.edu/articles/glaze_tech/simple_glaze_chemistry.html

http://www.pottery-magic.com/pottery/glazes/glaze-defects.htm

http://ceramics.sdsu.edu/articles/glaze_tech/simple_glaze_chemistry.html

http://www.duncanshearer.co.nz/glaze/glazechemistry.html 

http://en.wikipedia.org/wiki/Kiln

 

 

The Encyclopaedia Britannica: A Dictionary of Arts, Sciences, and General Literature

By Thomas Spencer Baynes

.  Published by H.G. Allen, 1888

 

 

 

 

 

Questions:

 

1. There are salt glazes, alkaline glazes, tin glazes.  What are their purpose and what other types of glazes are there?

2. Glazes are applied as liquids.  Are there other techniques such as adding powder or other material over the brushed on glaze?

3.What is the difference between glazes and glass?

4. Fired porcelain is smaller than the greenware.  What about earthenware and stoneware?

5. Blue and white cameos were a favorite theme for wedgewood pottery.  What type of pottery was used and what is the finish?

6.ramp, soak.  Is there a term for the cooling period?

 

 

 

 

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