Dremel 876 Specifications download pdf (Page 6)

Classifications of Clay Bodies

Two common methods of classifying clay bodies are

according to the firing temperature range of the clay and by

composition.

Pyrometric Cones

The evolution and history behind the development of

pyrometric cones is interesting – any practitioner of ceramics

or pottery should be well acquainted with their purpose.

Perhaps the best resource on pyrometric cones in the U.S.

would be the Orton Ceramic Foundation

(www.ortonceramic.com). A non-profit organization dedicated

to the ceramic industry Orton publishes many handbooks and

guides on proper firing and kiln operation. Their publication,

“Cones and Firing” is a mandatory read and reference for

every clay user. A brief overview of cones is all that is needed

at this time.

A pyrometric cone is a small pyramidal-shaped compo-

sition of clay, frit and binders that bend at a specific temper-

ature. They are the standard method for determining the

maturing temperature of a clay body. Just like the clay bodies

they monitor, cones respond to temperature, duration, and

atmosphere. As the firing progresses and reaches maturity the

tips of the cones begin to bend down, forming an arch. When

the tip of the firing cone reaches the bottom of the cone the

firing is complete.

The lowest cone number is a cone 022 which “bends” at

around 1080∞ F, while the “hottest” cone is a cone 42 – a cone

used in advanced ceramic industry and a temperature you’ll

never want to experience.

Clay and glazes are commonly classified according to their

firing range. Low-fire clays mature between cones 010 and 06

– referred to as the bisque-fire range; mid-range clays mature

from cone 4 to cone 7; high-fire clays mature at cone 8 to cone

11 – occasionally higher.

Clay Body Compositions

Earthenware – traditionally red or buff colored, these clays

are rich in iron. To offset the flux properties of the iron a good

earthenware body will include fair amounts of grog or sand –

to give the body structure – and fireclay or stoneware to help

improve the body’s ability to take higher temperatures without

bloating or deforming.

Another type of earthenware clay body that is more

“designed” than found in nature is the whiteware body. These

bodies general exclude the use of natural earthenware clays and

are comprised of talc and ball clays. These clay bodies are

often white (hence the name) and are popularly used for

dinnerware.

Porcelain

Porcelain is most commonly known for their whiteness

and for having some degree of translucency. They are pure

and vitreous, which gives them their inherent glaze, fit quality,

hardness, and durability. The whitest porcelains usually feature

up to 50% kaolin as the primary clay component with additives

such as bentonite or macaloid to help improve plasticity.

True bone china is so titled due to the addition of bone ash

(calcium phosphate). The calcium reacts with silica to develop

a very glassy material, giving the finished product the tradi-

tional translucent look.

Stoneware

These clay bodies use natural stoneware clay and/or

fireclay, with ball clay, kaolins, flint, fluxes and grog or sand as

additives. The choice of additives is truly an issue of material

design and use. For example, adding sand or grog gives more

structure reduces slumping. Clay users who want to hand build

or throw taller wares with thinner bodies may opt for this type

of body. Silica and feldspar may be added to help control the

maturing temperature and glass-forming characteristics of the

body.

Stoneware clay bodies produce durable ware and generally

react favorably to the kiln environment during reduction firings

(reducing or restricting the amount of oxygen in the kiln, which

causes carbon and hydrogen to build up inside the kiln, altering

the appearance of the clay and glazes used in such a firing).

Refractory

These clay bodies are used for making firebrick and kiln

furniture (A refractory material is one that can withstand high

temperatures). A hot-face fire brick might include up to 80%

grog and 20% plastic fireclay or low-iron ball clay. While

there are many types of refractory applications, the higher

temperature bodies will require reduced amounts of flux.

*The above information was compiled from

“Clay: A Studio Handbook” by Vince Pitelka.

Determining Which Clay is Best for You

When looking for clay, compare the temperature range

used for the glaze. For example, if using Mayco’s Series 2000,

which is 06-04 glaze, look for an 06-04 clay body, which is

Earthenware clay. This will ensure the best glaze fit.

Suggested Clay Bodies for Teachers

While any earthenware clay is ideal for teachers, white

earthenware is the most time-efficient clay to use - since

teachers do not have the time to fire long. Although red

clay gives beautiful results, many teachers do not like iron

in their classrooms or on students’ clothing.

It is easier to achieve bright glaze colors with white

earthenware. This clay body will work for all grades and

projects. A slight addition of grog can be helpful to aid in

crack problems and the capacity of the clay to take on a

variety of forms.

While earthenware clay is suggested, it is important to

remember that they do not totally seal or become imper-

vious to water. Earthenware clay absorbs moisture even

after fired to temperature and is not weatherproof or frost-

proof. When left outside and exposed to extreme tempera-

tures and weather conditions, glazes applied to Earthenware

clays can craze and crackle.

If someone is looking for weatherproof clay, stoneware or

porcelain clay are better choices as the body is vitrified after

firing. These clays are typically fired to cone 6 or higher.

They can be used outdoors and for functional uses.

Clay

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