Mrs. Hasselbeck
Ceramics I
Spring Semester 2014-15
Homework #3: Properties of Clay and Types of Clay
___ / 30 POINTS
PROPERTIES OF CLAY AND TYPES OF CLAY:
There are three essential properties that make clay different from dirt. Clay, in order to be useable, must have these properties. As
discussed in your vocabulary homework, these are plasticity, porosity, and the ability to vitrify.
PROPERTIES OF CLAY:
#1: PLASTICITY:
Plasticity logically should be the first property discussed, because it has to do with clay in the wet stage. To be useable, clay has to have the
ability to hold its shape or form while at the same time be pliable (bendable) enough to be moved by the potter’s hands. This is plasticity,
and it’s determined by the size and shape of very fine grains or particles of clay called platelets. (These are shaped like plates or disks)
When a potter is working with the clay, the platelets slip and slide on each other and align themselves like bricks in a wall. (Compression of
these platelets adds strength to your clay when building). If the platelets are very small and very smooth, obviously they will be easier to
push into a specific form. It’s interesting to carry this idea of smaller and smaller and smoother and smoother to the point where the
platelets will not stop slipping and sliding. There is a type of clay called bentonite that does just that. Bentonite is too plastic- it’s like silly
putty and cannot be used by itself to make pottery, although it is often used as an ingredient in prepared clays to make them more plastic.
The water of plasticity is an essential part of clay’s ability to be plastic. Obviously, a dry substance isn’t going to move around and stick
together at the same time. Water is added to make clay both slippery and sticky. Too much water and you have goo; too little water and you
have powder. It’s an essential skill as a potter to learn when just the right amount of this water of plasticity is present in clay.
Besides water or bentonite, organic matter is another ingredient that makes clay slippery and adds plasticity. During geologic weathering,
materials like decayed leaves, roots, and grasses have been ground up over the centuries and mixed with the clay particles. The acids
formed by this decaying organic matter break down the particles and make the clay more plastic.
#2: POROSITY:
Porosity is the second property of useable clay- the ability to dry without cracking. Remember that some clays are too plastic, like
bentonite. The platelets in bentonite are too fine and too smooth and closely squeezed together to let the water of plasticity evaporate
without cracking the pot. This is what happens when a mud puddle dries. Instead of drying into a smooth flat surface, most mud puddles
dry forming cracks in all different directions. A clay, to be useable to the potter, must be porous (or coarse) enough to let the water escape
evenly from all parts of the ware. If you have a clay that is not porous, this can sometimes be fixed by adding silica sand or grog (clay that
has been fired and ground into a powder) to increase porosity. Grog also helps to add strength during building and firing.
#3: VITRIFICATION:
Vitrification is the third important property of useable clay. Vitrification is the process of becoming glasslike. Although clay products never
become absolutely or fully vitrified (as they would melt into a puddle) it is necessary that the clay become hard (or almost vitrified) at a
reasonable temperature. Any substance will melt at some temperature, but most materials become soft and deform before they melt. The
ability of clay to hold its shape and not sag or slump in the primary melting stages sets it apart from other materials. A clay pot will melt like
a pancake it it’s fired too high. Knowing when the clay particles have melted enough to be on the verge of distortion then turning off the kiln
is vitally important! Vitrification is no less important than plasticity or porosity- it is mentioned last only because it is the last stage in
pottery construction.
THE 3 GROUPS OR TYPES OF CLAY
There are three general groups of clay: 1) earthenware, 2) stoneware, and 3) porcelain type clays. These groups are based mostly on the
ability to vitrify (become glasslike) but also to a smaller extent on their plasticity and porosity. Earthenware, stoneware, and porcelain are
the general groups, but there are hundreds of subgroups. There are numerous varieties of manufactured clay bodies... each a specific
recipe to produce greater control of the three essential properties above, and even color.
All of our clays are bisque fired to cone 04 or 1926 º F, are then glazed, then refired to the appropriate glaze firing cone temperature.
EARTHENWARE
Earthenware is a low-fire clay (or low-firing temperature). It only partly vitrifies before it melts. The potter must know this and turn off the
kiln- or stop the firing process- when the clay is still very porous. If a red clay pot were fired too high to make it fully vitrified- it would turn
into a glass pancake. This limits the use of most industrial uses of earthenware to the manufacture of brick and tile (roofing/drainage tile).
In addition to causing the clay to be red in color, the iron in the clay acts like a flux (a melting agent) and melts at a lower temperature, and
therefore causes the clay to be low fire (it cannot be fired any hotter or it would melt). Due to their porous nature earthenware clays must be
glazed to be waterproof if they are intended to be used with liquids. (Glaze is a glassy layer melted onto a pot to seal the pores or decorate
the ware) Earthenware clay is also softer and not as strong or durable as the other groups of clay after firing.
STONEWARE
Stoneware clays are often buff (light yellow), brown, gray, reddish, or even close to white. Stoneware can become semivitreous or vitreous,
depending on when the potter turns off the kiln. At about 2190 º F (Potters call this cone 5) stoneware is semi-vitreous (this is the cone to
which we fire our stoneware glaze pieces in class) . This means the clay will absorb a little water, but not so much that most people would
even notice. At cone 10- or around 2350 º F, the same clay becomes in potter’s terms, vitreous. Absorption is nearly zero. The clay is harder
than steel- but more brittle, and will ring when tapped on the lip. We don’t fire to cone 10 here at school, because it is very hard on the kiln,
shortens the life of the kiln elements, and takes a lot more electricity.
PORCELAIN
Porcelain type clays are traditionally pure white, creamy, and without grog. They, like stoneware, are vitreous. Porcelain is fired
in a range from 2100 º to 2600 º F (cone 5 to cone 16), although 2350 º to 2400 º (cones 10 and 11) are most commonly
used. When fired, porcelain is very hard. Earthenware and stoneware clays are plastic and easy to work with. Porcelain is
usually a nonplastic (also known as short) clay body and somewhat difficult to handle. (Therefore we don’t use it here in class.)
Porcelain was originally developed in Asia centuries ago, and is well known for refined Chinese porcelain creations. That is one
reason that today fine porcelain dinnerware is still called “china”.
Mrs. Hasselbeck | CERAMICS I | HOMEWORK #2
NAME:
BELL:
DATE:
Check your reading! Supply the missing word(s) to complete the statements below.
1. There are three properties that make useable clay different than sand or dirt. These properties are:
a. ______________________________
b. ______________________________
c. ______________________________
2. The first essential property deals with clay in the wet stage: ______________________________
3. What is the quickest or easiest thing that can be added to clay to increase its plasticity?
______________________________
4. How can organic matter increase plasticity in clay?
________________________________________________________________________
5. Why might bentonite be added to a clay body? ______________________________________
6. What is the name of fine clay particles that are shaped like plates or disks?
______________________________
7. Why is compression of the clay particles helpful when working in clay?
____________________________________________________________
8. What is the name of the clay which is often added to other clays to increase plasticity?
______________________________
9. The second property of clay is the ability to dry without cracking… ______________________________
10. What can be added to clay to increase the porosity of it? ______________________________
11. This is the process of becoming glasslike, and is the third property of clay:
______________________________
12. What would happen to a clay piece that is fired too high? It would __________________________
13. When firing it is important to know when to turn off the kiln. When the pieces are reaching vitrification, we turn
off the kiln when the pieces are on the verge of ______________________________.
14. Although there are hundreds of subgroups of clay, there are only this many major groups of clay:
______________________________
15. The groups of clay are divided mainly on which of the three properties of clay?
______________________________
16. Manufactured clay bodies are blended specifically to control the following four things:
a. ______________________________
b. ______________________________
c. ______________________________
d. ______________________________
17. What cone and temperature do we fire all the pieces for the bisque firing? Cone _______________ and
______________________________ degrees.
18. This is the low fire clay we will be used: ______________________________
19. To make earthenware pieces waterproof, they should be: ______________________________
20. Are stoneware clays high fire or low fire? ______________________________
21. During which firing would vitrification (or even near-vitrifcation) of stoneware occur: during the bisque or glaze
firing? ______________________________
22. What is the type of high fire clay that we will not be using here in school due to its difficult nature of being nonplastic (short): ______________________________
23. What color is porcelain clay? ______________________________
24. What name is used to describe fine dinnerware made from porcelain today?
______________________________