Dremel 876 Specifications download pdf (Page 12)

•Do not place the rims of large plates and bowls next

to an element: this will heat the piece unevenly.

• Flat pieces should be placed on shelves above the

bottom two rows of elements for even firing. In this

instance, it's better to use two half shelves, leaving a

small 3/8” to 1/2” gap between them to promote better

heat and air circulation.

•Generally larger pieces go toward the top of the kiln.

•Large volume pieces should be centered in the

chamber with smaller pieces surrounding the ware.

• Flat objects are never set on end and should be set flat

on a shelf with no overhanging parts.

While electric kilns are designed for optimum heating

throughout, a heavily filled kiln will not fire accurately.

For a bisque firing, allow at least one inch between the

tallest piece and the shelf above it. During glaze firing, a

two inch or more clearance is best before the shelf is put

in place. At times when you have only a few items to fire,

compensate by "false loading" the kiln with a few extra

shelves and/or posts to act like other pieces in the kiln.

If you suspect that you are firing too fast and the ware

is not receiving the proper heat work, check your firing by

placing a shelf cone in the center of the kiln shelf and

cover it with an unglazed greenware or bisque bowl or pot.

After the firing, examine the cone to see if it has deformed

properly. If it did not, you are firing too fast and need to

slow the rate of firing.

Kiln firing has some basics. If you have switches that

read Off, Low, Medium and High or have numbering from

0-100, the following schedule will help you. The switches

are not turned up concurrently; this would cause an

overfiring in the top and an underfiring in the bottom of

the kiln. The switches are stepped up in stages to

compensate for the heat rising. An hour between turning

up switches is a common time frame.

While every loaded kiln and cone temperature is

different, you can modify the firing schedule. A heavier

load or heavier types of ware may take longer in the early

advancement of the switches. This timetable can be

modified to fit any kiln by any kiln manufacturer. The

manual for your kiln should give you guidelines for firing

rates and advancement of the switches and/or dials. After

your first few firings, the personality of your kiln will

dictate your schedule. Remember that over time the kiln's

personality can change; using shelf cones will help you

monitor this. The time table below may sound like and

extravagant scheme for firing, but it will give reliable

results, whether it is a low fire glaze, high fire glaze or

greenware.

If you have a computer programmable kiln, follow the

simple directions as it queues you. Generally the direc-

tions are on the side of the kiln. Remember that a 250˚ to

350˚ acceleration rate is best. Do not allow yourself to be

tricked into thinking that the computer automatically

performs correctly, always use shelf cones to verify the

heat work of the kiln.

Kiln Soaking

There are times when you need to "soak" the kiln load.

This is where the kiln, after it has shut off, is restarted and

the switches are turned back to medium for 30-45 minutes.

This can be achieved by pushing the plunger back in on the

kiln sitter. After the duration of this time, it is shut off again

by pushing down the hammer to turn off the kiln. If you do

not have a timer, this must be watched carefully. This can be

done with a manual or computerized kiln. Check your kiln

manual for more information.

Changes that Happen During Firing

It is important to understand what happens to the ware

in each firing. During the early stages of firing (at approxi-

mately 100˚-300˚), physical water is driven out of the clay

and glazes. Rapid temperature increase could trap water

molecules in the ware or glaze; a long slow advancement

period during this temperature range is quite beneficial. At

this stage, the kiln lid should be propped three to six inches

to allow the water vapor to escape.

As the temperature rises through the 500˚ to 1300˚

range, impurities in the clay and organic matter are burned

out of the ware. Chemically released water and gases are

driven out of the ware as well. Good ventilation is very

important during this time; if there is not adequate venti-

lation, glazes may become discolored during this stage.

At around 660˚ to 700˚, greenware transforms from

bisque and can no longer change back into a lump of clay.

This term is referred to as "soft bisque." The ware can still

be cleaned and handled, but does not crumble as before.

At approximately 1000˚, quartz inversion takes place,

turning the quartz in the clay into a crystalline structure.

During this time, pieces actually move in the kiln. As with

everything that is subjected to heat, the ware expands, subse-

quently shrinking as it cools. When you compare a

greenware piece to a bisque piece, the difference is the

“shrinkage rate" of the clay. With most whiteware, the

shrinkage rate is 4%-10%. Poorly balanced pieces can

topple during this stage. Using kiln wash on the shelves

helps the pieces glide on the shelves during this expansion

and shrinkage. This stage produces the most stress on the

piece and happens every time a piece is fired. This

conversion of the quartz should take place at a slow rate,

both during the acceleration and cooling.

Overfiring

All clays have a maturing point; once the temperature

Firing

Switches Settings

12 3 4 5 6

Lid Propped Lid Closed

Top Off Low Low Med Med High

Middle Low Low Med Med High High

Bottom Med High High High High High

Top Peep Hole Open/ All Others Plugged

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 ... 40 41

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Dremel 876 Specifications Page 12

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