An ancient “Greek” style firing was done in the Alpine on May 29th, 2014. The Alpine was selected for the firing since it’s an updraft and fuel burning kiln, making it our closest approach to what the Greeks used. It’s also a relatively small kiln and easy to fire. (Details about the Alpine are available under Kilns and Firing).
The kiln had to be started slowly, like a bisque, because the load was all terracotta greenware. Just the pilots ran for the first 45 minutes, keeping temperatures below 200 F to safely drive off moisture. Then the main burners were lit at a 0.3″ gas pressure, which is as low as you can go with the Alpine and keep a stable flame, for another 45 minutes. Finally, the kiln was set to candle overnight at 0.5″ gas pressure; 22/100 primary air speed; and a 2.0″ damper gap.
By 9 am the next morning, kiln temperature had risen to 1472 F (800C). At that point, reduction was started by increasing gas pressure to 2″, and closing the damper to 0.5″. These settings quickly produced a four inch blue flame at the flue, and faint two inch flames out both upper and lower spy ports. Temperature continued to increase slowly until the kiln reached cone 07 (945 C). This was our target “end point” – the peak temperature cited in the literature for 5th century BCE Greek kilns.
As cone 07 fell to ~ 2 o’clock, temperature was backed-off while staying in reduction. This is done to hold iron oxide in it’s black form until the slips cool enough to harden – preventing re-oxidation later. We began by backing off primary air and closing the damper to 0.5″; however, these settings caused the kiln to stall; temps even rose a little.
Eventually, we had to turn primary air completely off to get a reasonable cool down rate. With the air at zero, kiln temperature did continuously fall, dropping to 1500 F over the next two hours. A deep orange flame and some soot were visible at the flue during this time.
At 1500 F, the kiln was completely shut off, and the damper closed. From there, it cooled on its own, naturally, in an oxidation atmosphere. After 4.5 hours, the temperature had fallen to 1000 F; and after 7 hrs the kiln had cooled to 850 F.
The next day, the kiln was cool enough to open for a quick viewing, and the adjacent picture was taken prior to unloading. There were definitely some surprises and mixed results, which have suggested how we might proceed with future terra sigilatta source mixtures and firing profiles. Preliminary thoughts are discussed below.
Note the black color sealed into the cone pack (upper right), showing the effects of sustained reduction to below the temperature where cones re-harden .
Terra Sigilatta Source Mixtures
Six different Terra Sigilatta slips were made for the firing. The recipes for their source mixtures are numbered below, and then referenced in the Results discussion that follows. Unlike most slip recipes, the weight of water is included as an ingredient. The process for separating a Terra Sigilatta from its source mixture is available from many sources, which include Cushing’s Handbook, pp 32; and Studio Potter, Volume 11, No.2 –
………………………………………..Terra Sigilatta Source Mixture List
Slip 1: Val Cushing Terra Sigilatta :
………..[ 70% water plus 15% each of Calvert.& Redart clays, and 0.1% calgon ]
Slip 2: Slip 1, plus 10% red iron oxide (RIO)
Slip 3: Slip 2, plus 10% colmanite
Slip 4: Slip 2, plus 3% wood ash
Slip 5: Val Cushing White: [ 70% Water plus 30% EPK and 0.1% calgon ]
Slip 6: 70% water, 30% Calvert Clay, plus 2% Red Iron Oxide, and 0.15% calgon
………….( For example: Add to 14 Cups Water, 1400 grams Calvert + 100 grams RIO
……………and 7.5 grams of Calgon ).
The spherical bowl shown right is the best example of a Greek Black Figure vessel that came out of the firing. While in the greenware state, it was brush coated with Slip 1 over the entire outside surface. Then the floral pattern was applied with up to three brush coats of Slip 2. The background fired into a slightly satin iron red; while the floral pattern became faintly to solidly black depending on how many coats of Slip 2 were brushed on. The deepest black was three coats thick. Since the slips were nearly the same color before firing, it was hard to keep track of where the brush strokes began and ended.
The small carinated vase here has a geometric pattern (now barely visible) painted horizontally around the upper surface, ie: above the carination; and was burnished after applying the slips. The background was coated with Slip 3, which was expected to yield a good black coverage since it has additional flux in the recipe. It did melt better than Slip 2, but it didn’t quite produce full coverage. This may have been due to some of the slip being rubbed off by burnishing, or possibly too low a firing temperature.
The geometric pattern (done in Slip 2) fired into a black, slightly matte finish similar to the spherical vase above, which stands to reason since Slip 2 has no additional flux.
The adjacent closed form bottle is intended for use as a fragrance diffuser with aroma sticks. The entire surface was first painted with Slip 1, which fired into the expected satin red color. Then using Slip 5, Val Cushing White, a pattern of diagonal lines was applied along the foot, and a square frame design roughly in the center. Both of these correctly fired into an opaque, bright white.
Last, around the perimeter of the square frame, a series of small triangular shapes and dots were applied using Slip 3. Each element was brush applied with one coat, so they turned into a somewhat faint black.
This vase was entirely brush coated with Slip 2. The firing locked-in the black reduction effect over most of the surface, > 90% , and the color is consistent with the carinated vase results above. Slip 2 produces a somewhat fainter, more matte color since it has no additional flux. While this is a successful color, the consensus is to try a slightly higher peak temperature to see if it would gloss up and/or achieve fuller coverage. Both Slips 2 and 3 may well benefit from a little more heat or a soak at temperature.
No decorative pattern is discernible.
The large smoothly modeled, hand build bowl shown here is coated in Slip 1. In this case, Slip 1 created a cloudy “red and black” effect spread randomly about the surface. There is also a faint yellow-tinted pattern, visible on the inside base, that was applied with Slip 4, which includes wood ash in the source recipe.
In other pieces, Slip 1 produced a solid red surface, so this may indicate the kiln’s peak temperature was close to the point where Slip 1 sinters, sealing off some of the black iron but not all.
The final piece is the only successful Red Figure piece from the firing. It’s a tall cylindrical vase fully coated in Slip 5 (white), with the foot and rim additionally framed with two coats of Slip 6. Finally, a sequence of vertical, slightly serpentine, floral patterns were applied with Slip 6. Where thick (two coats) Slip 6 turned black, and where thin it remained red. The red is thin, so it appears somewhat pink due to the white background.
Several thoughts about the fired results led to ideas for the next firing :
1. Fire a little hotter – to cone 06 tipping instead of cone 07. Intent: Increase slip melting to improve the black reduction color.
2. Use a dip application for the Terra Sigilatta background instead of brushing. Intent: Smoother, more complete coverage.
3. Add 4% cobalt carbonate to Slip 1. Intent: Improve the black color.
4. Repeat Jerry Hesse’s slip plan (used on the spherical bowl) to make sure it remains stable at slightly higher temps.
And now…….here’s Jerry Hesse in front of the Alpine kiln holding the current winner for
……….Best Greek “Black Figure” Vase
of Spring 2014. If you’d like to try your hand with some of these slips or have your own ideas for another “Greek” firing (now planned for the Fall term), make a terracotta piece and let it dry over the summer. Do not bisque. Then try out some decorative slips in the Fall !
Be sure to put a slip of paper in your pot to identify it as a terracotta piece for the next special “Greek Firing.”