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Boron in Ceramics, Enamels, and Glazes

BETTER FIT, BETTER FRITS

Boron has been an essential ingredient in ceramic glazes and porcelain (vitreous) enamels for centuries. Borates initiate glass formation and reduce glass viscosity, helping to form a smooth surface and reduce thermal expansion. These qualities help to ensure a good fit between the glaze or enamel and the item it covers. Borates also increase the refractive index (luster), enhance mechanical durability and resistance to chemicals, and help to dissolve coloring agents.

Borates are also incorporated into frits to render them insoluble. The resulting material is then mixed with other materials, including water, and finely ground to make a suspension that can be applied to the surface of the desired substrate. After application, the substrate is dried and fired to fuse the glaze or enamel onto the surface. Frits also basically “premelt” the ceramic glaze before the firing process, helping to obtain a high-glaze gloss even when using very short firing cycles.

Boron in ceramic tiles

Glazes: Thin, glassy coatings that are fused onto the surface of ceramic substrates, such as wall and floor tiles, and tableware like bone china and porcelain 

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Boric oxide as a flux and former

In glazes and enamels, boric oxide reduces melting temperature and improves the fit between the glaze and the ceramic or metallic body. The resulting glaze or enamel is attractive, lustrous, and durable. Borates are typically used in glazes for two categories of ceramics: Tiles and tableware.

  • Tiles: Wall, floor, and sometimes clay roof tiles
  • Tableware: Porcelain, china, stoneware, and earthenware

Although borates are a part of many tableware glazes, 20 Mule Team® Borax boric oxide (B2O3) is primarily used in glazes for ceramic wall and floor tiles. Because the raw materials that supply this oxide are soluble in water, they must first be rendered insoluble by incorporating them into ceramic frits.

Boric oxide can form a glass on its own, but in glazes it functions as both a flux and network former:

  • The value of boron as a flux has been recognized for years, and it is a regular component of recipes for low-temperature (< 1100°C) glazes. In glazes with B2O3 contents below 12%, boron does not increase the thermal expansion coefficient, enabling the production of glazes that behave appropriately at the temperatures used in current manufacturing processes. Other fluxing oxides (eg, alkalis, alkaline earths) are network modifiers rather than network formers and so increase the thermal expansion coefficient. Lead oxide can be a former or modifier but is no longer used in ceramic tiles because of its toxicity.
  • In glaze technology, borates are the second-most important network former (after silicon). Boric oxide reduces surface tension, does not crystallize from melts, and tends to hinder the crystallization process of other phases. These effects are useful in the production of glazes with high gloss: Low surface tension produces a flat glaze surface, and most crystalline phases in a glaze reduce the surface flatness and gloss. Boric oxide increases the gloss or brilliance of a melt but does not increase the refractive index. Borate also has a strong solvent action on coloring oxides, and boron glazes are good bases for glazes that are colored by dissolved transition metal oxides.

Correctly used, boric oxide can also greatly improve chemical durability, mechanical strength, and scratch resistance. Many types of glaze would be impossible to produce without boric oxide; for example, glazes formulated for single- and double-fire wall-tile manufacture.

Definition

Boric oxide for porcelain enamels

Porcelain enamels: Similar to glazes but used on metal substrates such as pots and pans, household appliances, metal bathtubs, storage tanks and silos, architectural panels, and signs

 

FACT
Copper and aluminum can be enameled by using borate-based frits, but enamel frits are primarily used on steel and cast iron.

Frits for ceramics and enamels

The boric oxide content of a frit depends on the type of glaze for which the frit is intended:

  • For glazes that are used in ceramic-tile manufacturing, the higher the firing temperature the lower the frit’s B2O3 content
  • High temperatures or long firing cycles mean a high-heat work, which reduces the amount of boric oxide allowable in the glaze
  • Excessive boric oxide can result in pinhole defects
Definition
Frits: Glassy materials that are rich in silica (SiO2), obtained by fusing different crystalline materials at high temperatures (up to 1550°C) and then rapidly cooling the melt

Tile Manufacturing Processes and B2O3 Content 

  B2O3 Content in Frit (% by Weight) Firing Conditions (Temperature/Time) /th>
Wall tiles, traditional double firing 8 – 20  980–1000°C / 360–720 minutes
980–1000°C / 360–720 minutes
Wall tiles, fast double firing 4 – 10 1060–1080°C / 30–55 minutes
Wall tiles, fast single firing 3 – 6 1100–1120°C / 35–55 minutes
Stoneware floor tiles, fast single firing 0 – 3 1140–1180°C / 35–55 minutes

The popularity of traditional double firing has plummeted because of rising energy and labor costs; single firing is now the norm. The type of frit and borate that are suitable for double and single firing varies, as does the average boric oxide content of the frit. In double firing, which benefits from the use of sodium oxide (Na2O) in the frit, a sodium borate is the best choice. In single firing—especially for wall tiles— sodium oxide is typically not desirable, so a non-sodium borate is preferred.

Sodium borate helps to produce low-viscosity frits for enameling of metals—principally steel, cast iron, and aluminum. Enamel frits differ from ceramic frits since the properties of the substrate are quite different. Ceramic bodies are typically fired at temperatures above 1100°C and have a low thermal expansion coefficient. Enameled steel is fired at around 800°C and has a thermal expansion coefficient twice that of ceramic bodies. Therefore, enamel frits are much softer (less viscous) than ceramic frits and have a much higher thermal expansion. To achieve these qualities, enamel frits contain more boric oxide and much more sodium oxide than ceramic frits do.

FACT

Boron in ceramics

Optibor® boric acid is a common primary source of boric oxide (B2O3) for production of frits for single-fired wall tiles and for lead-free frits.

 

Boron Carbide

Neobor® borax pentahydrate’s purity; high, consistent level of boric oxide (B2O3); and 21% sodium oxide (Na2O) (a powerful flux) makes it a popular choice for frits for double firing and for floor-tile frits for which sodium is a benefit.

Boron in Ceramic vs Metal Frits 

  Ceramic Enamel (steel or cast iron)
Typical B2O3 content  5% (fast single firing)  14%
Typical Na2O content 0.5% 14%
Preferred borate Non-sodium Sodium
Predominant furnace type Continuous Batch
Fusion temperature 1500°C 1250°C
Firing temperature > 1100°C (fast single firing ) ~ 800°C
Quenching method Water Chilled rolls

20 Mule Team Borax Products

These 20 Mule Team Borax products are developed for use in the manufacture of glazes and enamels.

Borax Decahydrate

From detergents to dyes to adhesives, this mild alkaline salt does it all, particularly excelling as a buffering and fluxing agent. Learn More

Boric Oxide

Pure anhydrous form is ideal where boric acid is required without metals. A powerful tool in the production of specialty glasses, ceramics, enamels, and fluxes. Learn More

Dehybor

This hard, glassy, granular product is excellent when forming flux or glass, where it helps to increase yield and reduce energy consumption. Learn More

Neobor

With lower transportation, handling, and storage costs, this concentrated sodium borate is used in glass, fiberglass, cleaning products, and flame retardants. Learn More

Optibor

From reducing melting temperatures in fiberglass production to inhibiting corrosion in fuel additives, Optibor has a multitude of uses in numerous industries. Learn More

More about Borates in Ceramics, Enamels, and Glazes

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Borates in the Kitchen
The kitchen and utility room are two of the most borate-rich places in the home. Borates bring convenience, safety, and cleanliness to cooking and domestic ch... Learn More
Early Use of Borax in Ceramics and Enamels
The art of enameling began to take form in the early Byzantine era, but borax was not used in the frits applied to metals until the middle of the 18th century... Learn More
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U.S. Borax, part of Rio Tinto, is a global leader in the supply and science of borates—naturally-occurring minerals containing boron and other elements. We are 1,000 people serving 500 customers with more than 1,700 delivery locations globally. We supply 30% of the world’s need for refined borates from our world-class mine in Boron, California, about 100 miles east of Los Angeles.  Learn more about Rio Tinto

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