Say Hi To Alkalis And The Best Low-Fire Fluxes

Say hi to the weary best low-fire flux crew:

The Best Low-Fire Fluxes

In this list, you’ll find a handpicked selection of what I think are the best raw-material low-fire fluxes; they are all melters/melting agents/fluxes for low-fire ceramics and pottery. Not added to the list (of health or environmental concerns) are: Lead, Barium, Vanadium, Cryolite, or Fluorspar, though all are strong melters. Bismuth is a great melter and belongs to the list, but way to prizy to be used in production. For completeness, also adding 4 fluxing colouring oxides since they can contribute strongly to the melt.

The List ended up with a total of 9 different raw-material “families” or chemical elements. Split into two groups: The Melters and The Modifiers.

Boron B – got these kids: Borax, Boric Acid, Gerstley Borate, Colemanite, Ulexite.
Sodium Na – got twins: Sodium Carbonate (Soda Ash) and Sodium Bicarbonate.
The rest are only children:
Lithium Li Lithium Carbonate
Magnesium Mg Magnesium Carbonate
Potassium K Potassium Carbonate (Potash)
Strontium Sr Strontium Carbonate
Zinc Zn Zinc Oxide
Calcium Ca Calcium Carbonate
BismuthBi Bismuth Oxide.

Some metal oxides used for coloring also have strong fluxing capabilities:

Copper CuO Copper Oxide
Manganese MnO₂ Manganese Oxide
Cobalt CoO Cobalt Oxide
Iron Fe₂O₃ Iron Oxide (Iron especially in a reduction kiln atmosphere).

To write one line about each of the best low-fire fluxes on earth, and how they react around 950–1050 °C / 1742–1922 °F has to be a simplification, so here we go:

Boron – an important low-temperature glass former and flux, helps silica form a glass at temperatures around 900–1050°C and produce a stable melt; sources of Boron:
Borax – Supplies both boron and sodium, two strong contributions to a low-temp flux. Very active at 1000°C. Water-soluble.
Boric Acid – A “pure” Boron, useful for exploring the effect of boron itself.
Gerstley Borate – A natural borate material containing boron + calcium, a very effective traditional low-temperature flux, but its composition can vary.
Colemanite – A more defined calcium borate with less composition variation. A strong low-temperature flux without the sodium found in borax.
Ulexite – A naturally occurring sodium-calcium borate mineral, a powerful low-temperature flux around 900–1050°C.
Lithium – Lithium Carbonate – A powerful alkali flux.
Sodium – Sodium Carbonate / Sodium Bicarbonate – Fast and aggressive low-temperature flux.
Potassium – Potassium Carbonate – A powerful alkali flux, especially effective at low temperatures. Potassium-rich glazes have a richer, softer, and less sharply “alkaline” character.
Bismuth – Bismuth Oxide – Unusual, expensive, interesting flux that melts at really low temperatures.
Zinc – Zinc Oxide – Not a dominant flux but affects melting; also a crystal maker.
Calcium – Calcium Carbonate (Chalk) – A team player, not as strong at 1000 °C, but reactive with Boron and alkalis.
Strontium – Strontium Carbonate – Not the strongest flux alone but contributes to the melt with other fluxing agents.
Magnesium – Magnesium Carbonate – Weak low-temperature flux by itself; makes matte surfaces.
Copper – Copper Oxide – A colorant, but also a strong secondary flux at low temperatures.
Manganese –Manganese Oxide – A colorant, an active flux at 1000°C; substantial additions can produce a very fluid, oxide-rich glass.
Cobalt –Cobalt Oxide – A colorant and a powerful flux oxide, although cobalt is normally used in such small quantities that its contribution to the melt is limited.
Iron –Iron Oxide – A colorant; in oxidation, a relatively modest flux at 1000°C. In reduction, iron is a powerful flux. The kiln atmosphere effectively changes how strongly it behaves as a flux.

The Best Low-Fire Fluxes

Approximate temperature where the material becomes an active ceramic flux:

~700°C - Boric Acid B₂O₃
723°C - Lithium Carbonate Li₂O
~750°C- Borax B₂O₃ + Na₂O
~800°C - Ulexite B₂O₃ + Na₂O + CaO
~800°C - Sodium Bicarbonate Na₂O
817°C - Bismuth Oxide Bi₂O₃
~850°C - Sodium Carbonate / Soda Ash Na₂O
~850°C - Gerstley Borate B₂O₃ + CaO + Na₂O
891°C - Potassium Carbonate / Pearl Ash	K₂O
~900–1000°C - Colemanite B₂O₃ + CaO
~900–1000°C - Copper Oxide CuO / Cu₂O
~950–1000°C - Zinc Oxide ZnO
~950–1000°C - Manganese Oxides MnO
~950–1000°C - Iron Oxide FeO (reduction)	
~1000°C	- Cobalt Oxide CoO

This post is about the earliest glaze melters used in ceramics for practical use. But when two or more materials react during firing, they can begin forming a liquid at temperatures far below the melting points of the individual materials. This is known as eutectic melting. At what temperature a material starts to act as a flux by itself, is only the first part of the story.

Ceramic materials rarely melt in isolation: Sometimes the right combination and proportion of materials can be more important than the melting temperature of any single ingredient, and a material that seems unimpressive on its own may become remarkably active when combined with two or several fluxes together.

But that is another post (coming).