Specific Gravity of a Glaze

Glazes are mixtures of clays, minerals, and water. Their Specific Gravity (SpGr) tells you how much dry material is in the glaze; commonly thought of as how “thick” the glaze is. A value of 1.4 indicates a pretty thin glaze, whereas 1.7 is pretty thick. Terra Sigilatta is very thin, it has an SpGr of about 1.15; while casting slips are thick, measuring out around 1.75.

Checking the SpGr helps avoid over or under thickness while glazing. Over thickness, of course, is one of the big reasons for glaze runs. Under thickness often leads to dark, muddy results. Having the right SpGr is important to getting consistent glaze results.

A good way to measure SpGr

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Figure 1. Set-up and TARE

1) Place a dry, empty 100ml graduated cylinder (located in the Glaze Room upper cabinet) on the studio scale with enough added weight so that the total adds up to about 0.5 pounds. See Figure 1; click to enlarge. The added weight is needed because the minimum weight for scale accuracy is 0.44 lb.

2) Push the TARE button located on the right side of the keypad. The weight displayed will then drop to zero.

3) Carefully pour 100ml of glaze into the cylinder, and place it back on the scale. Wipe off any excess water or glaze; these create sources of error.

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Figure 2. Multiplier converts lbs to SpGr.

4) Use the keypad to enter the multiplier 4.54. This converts the glaze weight into grams and divides by 100 in one step. The Specific Gravity (SpGr) then automatically displays to the right as shown in Figure 2. In this case, it’s 1.41.

5) Compare the result in step 4 to the target SpGr written in the Student Glaze Book (Located in the 2nd drawer of the Glaze Room filing cabinet.) Each glaze is listed on a separate page, and cataloged in alphabetical order.

If your glaze is more than 4 or 5 points too thick or thin, discuss how to proceed with your teacher or a studio potter !  Don’t try to adjust the thickness of the glaze yourself.

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Figure 3. Student Glaze Book

In this case, the glaze is Jasper Tan with a target SpGr in the glaze book of 1.42, noted just below the glaze name, so the thickness is near perfect…..just 1 point on the thin side.

At CAW we bisque to cone 06 (1823F), and adjust our glazes to a SpGr usually between 1.4 and 1.6 .

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Dry Mix-to-Water Ratio:

It’s sometimes handy to know the ratio of dry ingredients to water in a glaze. For example, a SpGr of 1.5  does not  actually mean there’s 50 g of dry mineral in each 100 g of water. That mixture would weigh 150 g, but the volume would be larger than 100ml. That’s because the water alone takes up 100ml of space; the dry mix adds more volume.

A good way to find out the amount of dry mix in a glaze.

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Weigh one cup of glaze, and let the water evaporate.

1. Place an empty, dry container large enough to hold one measuring cup of glaze onto the studio scale, weigh it, and push the TARE button.

2. Pour one measuring cup of glaze into the container. The weigh shown will be the weight of 230 milliliters of glaze. Write down the weight.

3. Let the cup fully dry out (takes a few days), then weigh the cup again. That weight is the weight of the dry ingredient alone, while the difference in the two weights is the water weight.

Ballpark numbers for CAW glazes are between 250 to 300 grams of dry mix per measuring cup (230 ml) of glaze. Since there’s four cups in a quart and four quarts to a gallon, you need 4000 to 4500 grams of dry ingredient per gallon of desired glaze. That means there’s 35 to 40 lbs of dry mix in each full bucket of glaze. For the glaze in Figure 3, Jasper Tan , ingredients are calculated in the book for making 1, 2, 3, & 4 gallons of the glaze.

Interesting Note:  The weight of the dry minerals exceeds the weight of the water in a given volume of glaze !

 

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Greek Terracotta at the MFA

Next time you’re up in Boston, think about visiting the newly organized Greek exhibits at the Museum of Fine Arts. There’s over 200 items on display, newly arranged by theme into three galleries: Homer & the Epics; Dionysus & the Symposium; and Theater & Performance – themes representing the heart of Greek culture.

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Calix Krater, Terracotta, Red Figure, c 470 BCE

 The Homer gallery is devoted to works illustrating the Iliad and Odyssey, including an overview of the Trojan War through scenes on six terracotta vases, as well as marble sculptures and clay statuettes. One drinking cup depicts Helen of Sparta preparing to leave Greece with Paris, prince of Troy, ultimately leading to one of the most famous wars in history. Shown here is a classic red-figure vase from the gallery depicting soldiers in battle during the fall of Troy.  

The Dionysus gallery commemorates the god of the grape harvest, and the Athenian tradition of good conversation sparked by good wine. A silver case displays wine-related pieces used in symposiums, as well as a cup sculpted into a donkey’s head c. 480 B.C.

The third gallery highlights Performance. Terra-cotta pieces here illustrate scenes from lost dramas and comedies; only about 30 of which remain from the classical period. One vase shows Achilles striking down the Greek soldier Thersites, likely inspired by a lost Greek play.

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Neighborhood Music School

OLYMPUS DIGITAL CAMERAOne Thursday morning during Spring term, children from the Neighborhood Music School came over to the pottery for a visit !

They were very impressed with the studio, as you can tell from their enthusiastic Thank You note shown here…..

Samantha Wong and Anita Griffith are specifically mentioned in the salutation !

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Go Greek !

Euphronius Kater; 21.7″dia x 18″h; Terracotta; Greek (Attic); Red Figure; 515 BCE

A treasure of antiquity, the Euphronios Krater, commemorates the death of military commander, Sarpedon, stricken during the Trojan war. This vase, and others like it depict the poetry of Homer as well as the many interactions of mortals with the gods. They were prized possesions awarded to olympic athletes and for acts of heroism. You can see a number of fine examples by visiting the Yale Art Gallery Hall of Antiquities (first floor, old building)There’s really no substitute for examining actual pieces, first hand ! Pay particular attention to the surface burnishing, linework, and dimensionality of the figures.

You’ll no doubt wonder how the ancient Greeks made and decorated such beautiful, commemorative ware.  The actual 2500 year old process has been lost over the centuries; but you can learn about what archaeologists, art historians, and potters have done to rediscover it in Anita Griffith’s Thursday morning session (9am to noon). The group is working with the same type of clay (ie: terracotta) that the Greeks used, and special black and red slips known as Terra Sigilatta.

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Amphora, 10″dia x 16.4″h; Terracotta; Red Figure; Greek (Attic); Berlin Painter; circa 490 BCE

Usually CAW terracotta is fired in oxidation with commercial low fire glazes. However, the Greek potters are believed to have used a single, three-tier firing method involving periods of oxidation and reduction to obtain the unique red and black figures. No written records survive, so nobody knows for sure !

Anyway, in a spirit of discovery, Anita’s class is working with slip recipes expected to yield the correct colors just as they did for the ancients. They’ll be once-fired to a low temp (~ 950 C) in a downdraft kiln with selected periods of reduction targeted to recreate those world renown effects.

Join in on the fun !  If you’re interested in trying out some of your own ideas for the upcoming “Greek” firing ( now planned for May 29th ), make a terracotta piece and try some of our greenware decorative slips !  Be sure to put a slip of paper in your pot to identify it as a piece for the special “Greek Firing. “

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Greek Firing Results

Firing Description
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. 

photo 1The 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 PotterVolume 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 2Red 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 ).

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Fired Results:

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Bowl; Sperical profile; Terracotta; 5″ dia x 3″ h ;
Greek Low Fire Reduction; Black Figure.

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. 

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Vase; Terracotta; 5″ dia x 3.5″ h;
Greek Low fire Reduction

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.

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Fragrence Diffuser; Terracotta; 5″ dia x 3″ h;
Greek Low fire Reduction; Black & White highlights

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. 

 

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Vase; Carinated closed-form – shallow rim ; Terracotta; 5″ dia x 3″ h ; Greek Low Fire Reductn.

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. 

 

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Bowl; Slab-built,sculpted; Terracotta; 10″ dia x 5″ h; Greek Low Fire Reduction; Black background.

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.

 

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Cylindrical Vase; Terracotta; 4.5″ dia x 10″ h; Greek Low fire Reduction

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.

 

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Greek Black Figure Vase
First Prise
Spring 2014

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.”

 

 

 

 

 

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Meet A Potter

Find out more about CAW potters and their work !

Visit our Meet A Potter page today for the latest pics and interviews !

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Totem Poles For CAW

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Looking at the front garden on a snowy winter’s day !

Studio potter Nancy Silipote hopes to create a new look for the garden area in front of CAW. Her vision is to install a group of vertical ceramic assemblies, or what you might call Totem Poles, emerging from the raised bed behind the Tiny Gallery obelisk. Having such freestanding poles, which were first encountered by European explorers of the Pacific Northwest, would continue a long history of monumental sculpture in America.

Nancy’s plan calls for a permanent installation of three totems of different heights looking out over Audubon Street. The totems would consist of individually stacked pottery forms of different colors and shapes assembled on top of one another and randomly placed onto each pole.

Totem Pole Mockup

Totem Pole Mockup

See our example mockup near the pottery door.

Nancy was inspired for the project while reading an issue of Ceramics Monthly years ago, and recently decided that now was the time for action. (Note: We have CM copies routinely available in the studio for anyone to browse.) The individual members or elements of each pole are currently being made by Studio Potters and students in the pottery. Any shape, form, color, or surface is acceptable; in fact, the more diversity the better !

You can be part of the project, too ! Just make a piece with a through-hole size of one inch in diameter (when wet) so that it can be slid onto the stainless steel center shaft. Let any studio potter know when it comes out of the kiln, and we’ll add it to the stack !

Become a part of CAW history, create your own piece for the CAW Totem !

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