The Instruction Manual is now updated and is marked February 2013. Also, by popular demand, a file called “Shrinkage Chart for Hadar’s Clay” has been added to the right-hand pane of this blog. It includes shrinkage charts for both flat strips and rings.
Here are some of the changes to the manual:
Changes in Firing Schedules
1. Mid-fire pieces, under 100 grams total, can be fired in phase II for 1 hour only.
Please note: 2 hours and 100 grams are just convenient round numbers. It is important to understand the principle: the bigger the amount of metal fired in a single batch, the longer it takes the heat to reach all of it. Also, the heat will reach the outside area of pieces quicker than their inside area. Therefore, a thick piece, even if it weighs less then 100 grams, may need more than 1 hour to sinter.
2. Low-Shrinkage Steel XT on its own does not require pre-firing, and neither does Low-Shrinkage Steel XT combined with copper and Bronze XT. For these, fire 2 hours at high-fire schedule. Copper alone, Bronze XT alone, and the two combined, do require pre-firing, even at high-fire schedule. Any firing at low-or mid-fire schedule requires pre-firing.
Firing Repair Pieces
If the amount of added clay is small, repair firing does not require pre-firing, regardless of the firing schedule. One hour of firing should be enough.
Pre-firing on a stove-top vs. pre-firing in a kiln
Pre-firing can also be done in a kiln, with the piece resting on carbon but not covered by it. Ramp at full speed to 1000°F/538°C (brick kiln) or 1100°F/593°C (muffle kiln). A few minutes after the kiln has reached the target temperature, check the pieces. When they are all black and the smoke is gone, proceed to phase II. This may be a good solution wherever using gas is not allowed. However, the process is longer; it takes the kiln longer to reach the temperature required for the binder to burn out. Also, since the heat does not come from beneath the firing vessel, as it does on a stove-top, it takes longer for the binder to burn in the bottom parts of pieces (those that touch the carbon).
It is recommended to fire on top of carbon and not on a kiln shelf so pieces do not have to be touched when being transferred to the kiln for the second phase. At this point they are both hot and highly brittle.
The shape of the firing vessel
Round vessels seem to improve the sintering results. When firing in a circular box, there is no need to arrange pieces for optimal exposure to the firing elements. Pieces can be fired in the middle of the kiln, and in a front loader they can be fired in the front (close to the door) as well. The part in the manual that discussed avoiding certain areas in the kiln has therefore been omitted.
The advantage of pre-firing in open air
When pieces are pre-fired in open air, whether on a stove top or a kiln, they do not require a space of ½” between them when going into the kiln for phase II. Pieces can be fired very close to each other, as long as they do not touch. There is also no need to separate big pieces from small pieces. Once the binder is gone, it makes no difference. The part that discusses arranging pieces in the firing vessel has therefore been omitted.
In the firing schedules, “Ramp at 1800°F/1000°C” has been changed to “Ramp at full speed.” In most kilns, full speed is 1800°F/1000°C. However, if your controller allows a faster ramp, you can also ramp at 2200°F without causing any damage.
Firing in Cages
The part that discusses firing steel clays in cages has been omitted. Except for rings, there is no need to fire in cages. (As for rings, instead of using a cage, you can just place a piece of fiber paper under and on top of the ring shank). Why has this part been omitted? The cages were meant to prevent distortion when firing high-shrinkage steel clays inside carbon. Since Low-shrinkage Steel XT has been introduced, this distortion no longer happens and the cages are no longer necessary. For structural purposes, Low Shrinkage Steel XT is the only recommended type. The other types of steel clay are best used in combination with other metals.
Furthermore, cages introduce air pockets in the firing vessel. My experiments show that an excessive amount of air pockets inside the carbon results in serious cracking of pieces.
As for the shrinkage chart:
When you look at the shrinkage chart, you may be surprised to see that low-shrinkage steel XT shrinks by 28%. It may seem less confusing if you consider the following:
1. A shrinkage rate of 28% occurs at high-fire schedule. Structural pieces of steel clay cannot be fired at lower schedule, but small amounts embedded in other clays can. At lower schedules, Low-shrinkage Steel XT shrinks less than 28%, but because it is interlocked with other clays, there is no reliable way to measure the exact shrinkage rate.
2. Shrinkage is measured by volume (length x width x depth). If each one of these dimensions were to shrink by 28%, the overall shrinkage rate of the total volume would be much higher. The actual shrinkage of each dimension is a lot smaller than the total shrinkage rate by volume.
To make it simple, let’s consider a 2-dimensional object, like a square.
The edges of the bigger square measure 20 mm. After reducing the area of the square, the length is 17 mm. That means that while the area of the square shrank by about 28%, each edge shrank by only 15%.
In reality, though, even if we make “flat” pieces, they are still considered three-dimensional, because they have some thickness. Based on experiments that I have made with flat strips, the actual shrinkage of each dimension is 10%-12%.