Cosmic Pots: The “Goldilocks Glaze”

Thirty-four years ago, astronomer and Cosmos host Carl Sagan made his famous claim:

“If you wish to make an apple pie from scratch, you must first invent the universe.” – Carl Sagan. “The Lives of the Stars.” Cosmos: A Personal Voyage. PBS. 1980.

Sagan could have been talking about making anything from scratch. His goal was to convey that everything on earth, everything in the universe, is made up of precise combinations of the most basic elements, and those elements were created in stars’ nuclear cores. We could also say, “If you wish to make a pot from scratch, you must first invent the universe.”

These star-forged elements combine to form all the components of ceramics: the different strains of clay (silicon and iron), the water used in throwing (oxygen and hydrogen), the arboreal ingredients of glazes (calcium), and even the potter himself (carbon). Entire books could be written focusing solely on one of these ceramic elements.

copper red glazes pottery robert tichane cherrio potteryCopper, for example. Copper red glazes have been meticulously pursued and produced since the fifteenth century in China. The new host of Cosmos, Neil deGrasse Tyson, often analyzes the concept of a “Goldilocks planet” – a planet which has the precise conditions for possibly sustaining life. A successful copper red glaze is a “Goldilocks glaze.” Everything in both the recipe and the firing must be perfect.

Joel Cherrico Pottery, Copper Red Glazes, Gas Kiln Firing
Caution: No room for error.

As Sagan and Tyson have taught us, science is found in everything we do. Baking an apple pie from scratch, developing a new drug, and mixing and firing glazes all rely on experimentation, creativity, and chemical reactions. A potter doesn’t need a degree in chemistry, but he uses some pretty cool science to produce copper red glazes.

Copper Red Pottery, Stoneware Wheel Thrown Mug, Cups, Handmade Pottery, Handmade Ceramic Pottery, sku 427, Image 5Nowadays, gas-fired kilns produce the best conditions for copper red glazes, but ancient Chinese potters created their beautiful pieces using only wood-fired kilns. Many potters do not have regular access to gas- or wood-fired kilns, and use electric ones instead. Electric kilns eliminate the need for constant temperature monitoring, but they are unable to create the atmosphere copper red glazes require.

48 x 40 in. wall poster for Handmade Grounds
Hello in there!

Copper red glazes need to be fired to a temperature called “cone 10.” This photo shows three cones (small pieces of clay), set up inside a gas-fired kiln. Each of these pieces is made from a different factory-produced type of clay formulated to melt at a certain temperature. A device called a pyrometer can be used to measure the temperature of the air inside the kiln, but what really matters is the temperature of the clay, hence the use of cones. When cone 10 melts, the potter knows the clay is roughly 2345 °F.

Handmade Pottery Ceramic Copper Red Bowl, Wheel-Thrown pottery, Handmade Stoneware, SKU 445, Image 3

Stoneware Wheel Thrown Mug, Cups, Handmade Pottery, Handmade Ceramic Pottery, sku 426, Image 1Even inside the same kiln, the atmosphere unavoidably varies. The pots below all had the same glaze and firing, but were placed in different areas of the kiln.

Copper Red and Green Glazes, Joel Cherrico Pottery Cups  The green color on the right also occurs when firing a copper glaze in an electric kiln.

Handmade Pottery Stoneware Mugs, Wheel-Thrown pottery, Handmade Stoneware, SKU 438, Image 1Glazing Pottery, Copper Carbonate Stain

Color, just like copper, depends on the stars. Light from our sun strikes objects on earth, and those objects absorb some wavelengths of light and reflect others. The wavelengths they reflect are the colors we see. As Tyson puts it:

“Color is the way our eyes perceive how energetic light waves are.” – Neil Degrasse Tyson. “Hiding in the Light.” Cosmos: A Spacetime Odyssey. Fox. 2014.

Thankfully, potters did not have to create the universe to make pots from scratch. Their ingredients are already present in the cosmos, swirling in the air and lurking in the earth, waiting for them.

Handmade Ceramic Pottery, Joel Cherrico Pottery, Copper, Cobalt, Iron, Glazes, Pottery

Chuquicamata Till Niermann
Chuquicamata Copper Mine in Chile. (Image: Property of Till Niermann, located in the Wikimedia Commons.)

Glaze Chemistry and Alchemy

Pottery Alchemy, Alchemy Definition, Joel Cherrico Pottery, 2013

The New World Dictionary, Copyright 1967

In many ways, the work of the modern potter mirrors the work of the ancient alchemist. Potters blend earthly materials like clay, stone, and ash, into complicated glaze mixtures. Then through fire, these base substances transform into precious works of art. With glaze chemistry, and one part modern alchemy, potters turn the natural elements we once took for granted into the treasured artifacts we display in our homes and galleries.

It’s interesting to see how much the glazing, alchemy, and human life relate to each other. Bernard Leach, author of A Potter’s Book, helps us understand glazes by relating them to the body. He says most glazes have 3 main parts -the blood, bone, and flesh. Here’s how they work:

1.) Fluxing agent  or “life blood of the glaze” – causes the glaze materials to melt and flow together in the kiln firing.

2.) Refractory or “bone of the glaze”  resists heat and melting, providing structure and strength to the glaze body.

3.) Glass Former or “flesh of the glaze”  creates complexity, depth and unique qualities.

(page 133-134)

Similar to Bernard Leach, the early alchemists fused their chemical efforts with the body. Calling their experiments the Magnum Opus, or “Great Work,” these men searched tirelessly for the right chemical concoctions that would enrich life or prevent death. In some ways, full-time potters do the same through glaze chemistry. They are constantly searching for that perfect potion that will immortalize a clay body and turn sand, water, and ash into gold.

These 2 books, by potters John Britt and Phil Rogers, gave Joel the necessary skills to develop that perfect glaze surface, but like the early alchemists, he’s still searching.

Ash Glazes, Phil Rogers, Joel Cherrico Pottery, 2014    John Birtt the complete guide to high-fire glazes, Stoneware Pottery

Like alchemy, glazing is often a fiery, messy, and sometimes toxic process. The kiln releases CO2, the powdered glaze materials are dangerous inhalants, and the heavy metal colorants cause skin irritation. Joel mixes all his glazing in an old boat shed. This dirty, dark laboratory gives him 24 hour access to glaze experimentation, providing the perfect amount of chaos to create beautiful works.

Pottery Alchemy, Joel Cherrico Pottery, Glaze Mixing, Cone 10 Stoneware, 2014  Joel Cherrico Pottery, Pottery Alchemy, Glaze Mixing, 2013 Pottery Alchemy, Glaze Layering, Joel Cherrico Pottery, 2014  Pottery Alchemy, Joel Cherrico Pottery, Skutt Electric Kiln, 2014

Joel’s pottery has to be strong enough to be used in a coffee shop everyday. The Local Blend Baristas say they wash a mug up to 5 times per day, 7 days per week! With this in mind, Joel adapted the Nuka glaze to suit the stress. Traditionally a simple 3-ingredient mixture, Joel added more chemicals to strengthen the glaze surface, reducing flaws like pinholes and crazing while increasing durability and gloss. Here’s the recipe for all the curious potters out there:

Joel Cherrico Nuka Glaze, Cone 10 Recipe, 2014

Some potters spend their careers trying to find the right glaze mixtures. In next Friday’s post, we’ll delve into some of these mixtures more and explore the lure of pretty blue pottery.

Cobalt-based glazes, or what some potters call “cash-flow” blue glazes, have been mystifying both potters and customers for decades.
Cobalt-based glazes, or what some potters call “cash-flow” blue glazes, have been mystifying both potters and customers for decades.

“At my St. Ives workshop each summer we are asked by three visitors out of four for colour and yet more colour, blue and the more intense the better, is easily the favourite.”

A Potter’s Book, Bernard Leach, page 36