Article and research by Thomas Baurley, Technogypsie Research, August 7, 2017
Glazes on ceramics were recorded as far back as 2,000 years ago. Glazing involves firing the coating of a vitreous substance atop a ceramic item. This methodology is utilized to decorate, coat, color, and/or waterproof the ceramic item. Glazing alters the porousity of a ceramic item allowing it to hold liquids and strengthens its form. It is utilized on clay based ceramics, porcelain, terracotta tiles, bricks, sanitary ware, as well as stoneware. It is a popular finishing methodology for modern as well as historic ceramics, being a very common practice. Most of the traditional glazes are often named after their fluxing agents. Modern art glazes involve glass, plastics, metal, and other vitreous glazes for differing results.
- Ash Glaze
Plant or Wood Ash containing potash and lime is used as the fluxing agent to create a darker color.
- Feldsparic Glazes
Very common in Asian ceramics the primary fluxing agent is Feldspar.
- Lead Glazed Earthenware
This glaze comes out semi-transparent and shiny after firing, and dates back to over 2,000 years ago within Mediterranean, European, and Chinese cultures. Two common types are “sancai” and “Victorian majolica”. Firing is achieved at a mere 800 °C (1,470 °F).
- Salt Glazed Wares
Very common with stoneware in Europe, the fluxing agent is salt.
- Tin Glazed Pottery
Very common in the Near East, Middle East, and Islamic Culture, and later in European Culture, Tin glazes is a opaque white glaze that coats pottery with a tin based fluxing agent. Types include majolica, faience, maiolica, and Delftware.
The process of glazing requires partial liquification of the clay composites creating a ceramic flux and melding with other fluxing agents to create varying glazes and results. Most glazes consist of some form of silica or glass within them, and the fluxing agents will lower the high melting point of the silica to create the finish and strength needed. Sometimes Boron trioxide is added. Besides silica, other raw materials acting as fluxing agents can be minerals, chemicals, or metal oxides such as aluminum, alumina, calcium, potassium, or sodium. Alumina that comes from clays will solidify quickly the glaze so it doesn’t run or drip. In order to obtain varyig colors and finishes, some metal oxides are added as colorants to modify appearance such as with iron oxide, cobalt carbonate, coppoer carbonate, tin oxide, and/or zirconium oxide.
There are different processes used in applying glazes, ranging from brushing, painting, soaking, dipping, pouring, spraying, dry-dusting or atmospheric by inserting soda or salt within the kiln at high temperatures so the vapors chemically mixes with the silica or alumina oxides depositing glass or salt glaze ceramics. In the use of kilns, sometimes a small area of the pottery is left untreated so as not to create an error with the piece sticking to the kiln during firing. This is also accomplished by using kiln spurs, supports, or sticks to minimalize unfinished areas – but these too will leave small marks on the finished wares.
The glazed ceramics are often decorated with the glazes by applying an underglaze. These are done by “biscuit” firing (initial firing pre-glaze application and re-firing), application while the pottery is still in its raw pre-fired status, or by means of greenware. A transparent wet glaze is often applied over decorations. Decorations are done by grooving, cutting, imprinting, stamping, carving, stickling, dotting, impressions, drawing, painting, coloring, or imprinting. The pigments will fuse within the glaze and looks like it is covered over with a clear glaze. This is a popular underglazing process called “blue and white porcelain” displaying a impressive deep blue color formulated from cobalt carbonate and/or oxide that comes from England, Nederlands, Japan, and China. Overglazes are also produced by applying decoration on top of the glazes with the inclusion of a non-glaze substance like metals, leafing (gold leaf is popular), enamel, or layers of glass over the glaze utilizing low temperatures and makig a glassy appearance. The initial firing called “glost” is achieved after which the overglaze decoration is introduced, and a re-firing takes place creating a smoother texture to the ceramic or pottery.
History of Glazing
While over 2,000 years old, Glazing developed at a slower than expected rate through history, usually based on the elements included, and it took time for those materials to be discovered with realization what could be achieved by their inclusion. This evolved alongside the technological advances of firing and achievement of higher temperatures.
- 13th century BCE: Glazed bricks, Elamite Temple, Chogha Zanbil.
- 1600-1046 BCE: Proto Celadon glazed stoneware during Shang Dynasy
- 1049 BCE: Glazed bricks, Iron Pagoda, Kaifeng, China.
- 471-221 BCE: Lead glazed earthenware in China.
- 552-794 CE: Green natural ash glazes/Sue ware was found during the Kofun period, Japan.
- 8th century CE: Islamic use of glazed ceramics were uncovered.
- 13th century CE: Overglazes in use with red/blue/green/yellow/black flower designs
“In 1898 Edward Richard Taylor (1838-1912) and his youngest son William Howson Taylor (1876-1935) established Ruskin Pottery near Birmingham England. The Taylors named the pottery after writer and critic John Ruskin, whose ideals of quality and beauty they sought to embody in their works. The Ruskin Pottery style was based on hand thrown and hand-turned ceramic bodies with unusual glazes. The Pottery produced decorative vessels, tableware, buttons, and small glazed plagues called enamels, intended to be set in silver or pewter as jewelry. Howson Taylor continually experimented with new and sometimes difficult glaze techniques, resulting in four primary glases – souffle, luster, crystalline/matte, and high fired flambe – each explored in the following cases. When Ruskin Pottery closed in 1933, Howson Taylor refused to share his glaze recipes, writing, “Why let another firm make rubbish and call it Ruskin?”” Though the secrets were lost, the works on view in this exhibitio – drawn from a remarkable gift by Carl Patterson of over 200 objects – illustrate Ruskin’s dazzling range of glazes and shapes from its short but prolific history.” ~ Display at the Denver Museum of Art.
“Produced primarily from 1898 to 1914, souffle glazes were the company’s earliest technique. Souffle glazes were sprayed onto biscuit (single-fired) wares which were fired again after glazing. The spray application produces streaks or mottled effects. Ruskin Pottery founder William Howson Taylor described the finishes as ‘suggestive of the rich hues seen in rock pools at low tide.’ Many souffle glazes are monochrome, but some examples display contrasting colors or naturalistic stencilied motifs.” ~ Denver Museum of Art display.
“William Howson Taylor’s greatest glaze accomplishment was his high-fired flambe glaze, inspired by red glazes made in China during the 1600’s and 1700’s. This achievement required mastering the reduction-firing process, where the potter reduces the amount of oxygen in the kiln, which then causes copper-based oxides in the glaze to generate a range of colors, including cherry red, green, purple, and blue. The highly unpredictable process resulted in one-of-a-kind pieces.” ~ Denver Museum of Art display.
“Between 1905 and 1926, Ruskin Pottery produced luster-glazed wares. A metallic glaze applied over a colored glaze gives these objects a iridescent sheen. Ruskin’s luster glazes required five kiln firings, making them expensive to produce. Luster ceramics were especially popular in America, ad Ruskin’s kingfisher blue was the most desirable and unusual of these glazes.” ~ Denver Museum of Art display.
Crystalline / Matte
“In the early 1920s, Ruskin founder William Howson Taylor developed a glaze that took two forms: crystalline and matte. The crystalline glazes have distinct crystal formations: the matte glazes are not glossy and often include horizontal bands of colors dripping into one another. The pieces required only two firings and were sometimes molded rather than hand-thrown, making them more economical to produce than luster or flambe wares.” ~ Denver Museum of Art display.
Health and Environmental Concerns:
The historic use of heavy metals in glazes were determined to be a concern by 2012 in the United States for their widespread application in flooring, wall tiles, bathroom ceramics, kitchenware, tableware, and sanitary wares. Leeching of these heavy metals were found to be toxic when exposed to acidic or warm waters especially with chromium and lead in bioaccumulation resulting in heavy monitoring by government agencies.