Eutectic Melt -How Low Can You Go? (Ceramic FLUX)

Eutectic Melting & Glaze Experiments

Simon Harris’ “How Low Can You Go” is one ot the classics from the 80s, the song has nothing to do with glaze temperatures and ceramic fluxes, but it’s cool, right?

How low can you really go? And still melt a real ceramic glaze?

If you Google: “How to glaze pottery without a kiln?”, the answer is you can’t glaze-fire pottery at home without a pottery kiln.

But that’s wrong: You can glaze-fire pottery surprisingly low!

Even in an ordinary house woodstove!

Here, I do some practical experiments with what I call campfire glazes, which can all melt in a bonfire:
/glaze-fire-ceramic-in-the-campfire/

“A eutectic melt is a mixture of substances (for ceramics; metals, or salts) with the lowest possible melting point for the combination, melting all at once, rather than over a range of temperatures for each substance”.

Husk: “Animals – Lavtbrent fluxser.txt”

Low-temperature melting fluxes:

Low fire fluxes I have experimented with so far:

Extreme Low-Fire melting agents for ceramic glazes:

Start to flux: 
  723°C  - Lithium Carbonate
 ~750°C  - Borax (Sodium Borate) 
~750°C  - Boric Acid (do not contain Sodium)
 ~650°C  - Gersley Borate (active flux 750°C, fully molten 1000°C) 
  891°C  - Potassium Carbonate (Salt of tartar)
 ~800°C  - Sodium Carbonate/Bicarbonate (Soda Ash/Baking Soda)
(pure Baking Soda contains only Sodium Bicarbonate (NaHCO₃), while Baking Powder is not pure baking soda).
Bismuth Oxide (Bi₂O₃), I have not tested it, itssupposed to be one of the strongest low-temperature fluxes known, but also extremely expensive.

I desided to leave Lead out of the testing do to health conserns, same to Fluorides and Phosphorus materials; both can produce hazardousgaseswhen heated.   

~800–900°C - Lead oxide (PbO) 
~750-900°C - Cryolite (Na₃AlF₆) fluoride-based flux 
~700-900°C - Calcium Fluoride (Fluorspar) (CaF₂)contributes to eutectic melting

Many of the melting agents in the list melt first at 750 – 900 degrees Celsius, but practical experimentation tells another story; several will eutectisk melt in reaction with other fluxes, some even as low as 650°C.
In comparison, I have a friend making fantastic pizzas at 450-480°C, 650 is really low.

Here I am hunting for ultra-low glaze recipes based on Eutectic Melting:
/eutectic-melting-in-ultra-low-fire-glazes/

Tested with these low-fire melting agents:

Borax (Sodium Borate, Na₂B₄O₇·10H₂O)
Decomposes into Boron Oxide and Sodium Oxide. I buy it not as powder but small 0,5 mm “balls”, I give it a minute in the mortar to make powder. Too much Borax makes a glaze feel like brush-painting with sand when applied. When Borax is heated, it forms “crystal balls” that develop out of the glaze-surface and start “rolling off” the glaze: The Borax balls finally melt back into the glaze when hot enough. But the percentage of Borax you put in the receipt can be higher at the bottom of a cup than what’s left on the wall of the cup.

Boric Acid (H₃BO₃)
Quartz requires strong fluxing action, and Boric Acid alone is not highly effective at lowering its melting point. Around 500–600°C Boric Acid fully decomposes into Boron Oxide (B₂O₃). So, when Borax seems to melt better at low temperatures, it’s because Sodium Oxide is a stronger flux, though it needs a higher melting temperature: Sodium Oxide actively lowers the melting point of silica much earlier than boric acid, making Borax a more effective low-fire flux.

Lithium Carbonate (Li₂CO₃)
By far the earliest melter, but it is also a rare and expensive metal. I like to work with cheap, accessible “everywhere” materials, so I am reducing the amount where possible (preferably not using it). Still, Lithium is an important low-fire flux, and there are not many fluxes to choose.

Sodium Carbonate (Soda Ash, Na₂CO₃)
Cheap everyday compound. Behavior is quite different from borax or boric acid. Is the component used in Soda firing, where sprayed into the kiln to create a soda glaze. Sodium Carbonate stays mostly in the glaze at low temperatures. At temperatures above ~1000°C–1100°C, Sodium Carbonate will also start to vaporize from the glaze.

Potassium Carbonate (Salt of Tartar, K₂CO₃)
Also known as “Salt of tartar”, it got the highest fluxing temperature in this test. I have not tested it yet since it’s quite caustic, and I need a better mask first.

Extreme Low-Fire Glazes:

Based on my experiences:
Ceramic glazes can melt at extremely low temperatures. Several melting agents can, by themselves or in combination with other melting agents, make Quarts melt and form a glaze around 600-700 degrees Celsius. There seems to be a variety of visual and aesthetic alternatives even for extremely low-fired glazes, though the high percentage of low-temperature flux needed in the receipt, limits the amount of other glaze materials. It seems like a combination of several fluxes is the best way to achieve the desired melting behavior in a low fire glaze.

In the list above, it seems like the earliest melts happen at 723°C, but my firings don’t reach this temperature. I have measured clay in my bonfires to an absolute maximum at 600 – 650°C. This has to mean:

1. Melting agents react with each other, and eutectic melting occurs at a lower temperature, or:
2. Direct flame can locally (and for a short time) heat the glaze enough to start melting.

The next examples come from ChatGPT, I have not tested them. In my experience ChatGPT makes a lot of sense when asked about chemistry and reactions (but glaze formulas is not worth trying).

“These systems do not represent stable functional glazes, but extreme eutectic melt compositions used for experimental, historical, or proof-of-concept firing.”

1) Zinc Borate (commercial powder)
Widely used in industry as a flame retardant and low-melting flux.
Forms a glassy phase at relatively low temperatures.
Melting range: approximately 600–700 °C.

2) Vanadium–Borate Glass (V₂O₅ + borates)
Exhibits an extremely low melting point.
Vanadium introduces strong yellow–green coloration.
Toxic in larger amounts and requires careful handling.

3) Tin–Borate Glass (SnO₂ in borate glasses)
Also melts at very low temperatures.
Commonly used in the enamel industry.
Contributes to smooth, glossy surfaces.

4) Borate–Soda Eutectic (approx. 620–680 °C)
The classic “Egyptian faience effect.”

Recipe:
60% Borax frit P2953.1
20% calcined sodium bicarbonate
20% kaolin

Produces a glassy, glossy, slightly fluid glaze at surprisingly low temperatures.

5) Lithium–Borate–Soda Melt (600–650 °C)
An extreme low-melting system: lithium + borate + soda dramatically lowers the melting point.

Recipe:
40% lithium carbonate
30% boric acid
20% soda ash
10% fine silica (optional; increases hardness)

Flows at 600–650 °C; at 800 °C, it behaves almost like liquid water.

6) Colemanite “Lava Syrup” (650–750 °C)
Colemanite combined with soda forms a classic low-melting composition.

Recipe:
60% colemanite
20% soda ash
20% bentonite or kaolin

More stable than borax-based glazes, with greater body and less excessive running.