All Temperature Numbers Temperature Listing

932C (1709F)

Manganese compounds may begin to fume

Manganese metal fumes are toxic. It is difficult to verify the ceramic temperature where fuming could begin (welding is the context of almost all sources dealing with the topic). In welding circles, there is a significant level of concern regarding the dangers of manganese metal fume fever. The melting point of manganese metal is 1245C (around cone 7). However, in ceramics we use the dioxide form, MnO2. It decomposes to the monoxide form, MnO, well below typical firing temperatures. Theoretically, the MnO should be fairly stable. But in the real world material sources are crude and impure. Kiln firings are all about oxide interactions and reactions and the manganese has company - each MnO particle is surrounded, and vastly outnumbered by other compounds, especially fluxes, that may be trying to take away that atom of oxygen to reduce it to the metal form. And any production of carbon in the firing atmosphere can produce CO, hungry to do the same.

In our own testing, especially when mixed with frits, the crude granular ore of manganese melts vigorously around cone 6. And it generates gases vigorously after that.

Related Information

Decomposing manganese granular particles are causing this stoneware to bloat

This is a cone 6 stoneware with 0.3% 60/80 mesh manganese granular (Plainsman M340). Fired from cone 4 (bottom) to cone 8 (top). This body is normally stable to cone 8, but with the manganese it begins to bloat at cone 7! This is evidence that particles of manganese are generating gases as they decompose and melt at the same time as the body is vitrifying, these produce volumes and pressures sufficiently suddenly that closing channels within the maturing body are unable to vent them out.

How do metal oxides compare in their degrees of melting?

These metal oxides have been mixed with 50% Ferro frit 3134 and fired to cone 6 oxidation. Chrome and rutile have not melted, copper and cobalt are extremely active melters, frothing and boiling. Cobalt and copper have crystallized during cooling. Manganese has formed an iridescent glass.

Does manganese dioxide produce metal fumes at cone 6?

Black-burning clay bodies really enhance certain glazes. These two are examples: G3933B vs G3933D (similar to G3933E and G3933EF). However, these bodies are somewhat controversial. Some clay manufacturers are adding raw manganese - based on a belief that manganese dioxide (MnO2) does not decompose to produce metal fumes at cone 6. However, ChatGPT (see below) does not currently agree with this. As much as 10% MnO2 is needed to get color as dark as these samples. A kiln load of 50lb of pottery would have 5lb of MnO2! Other manufacturers use umber, reducing the impact 5-fold or more. However, this clay body, L4449J, adds 6% Mason 6666 black stain to Plainsman PR3D raw clay (+bentonite for plasticity). The result is a remarkably black clay body having no fume worries (which also means no glaze blistering). Is the extra cost and trouble of mixing this yourself is worth it? Maybe not. But there is a viable option: Slip casting. Black stain can be added to any clay body that can be deflocculated. And, ware is light so the cost might well be acceptable.

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