Manganese Oxide Classification and Description
There are six oxides of manganese, with the oxidation state of the manganese ranging from +2 through to +7, with one oxide bearing both +2 and +3 manganese oxidation states. Not all of these oxides occur in all ores, with the oxides used for pigmentation being in the lower ranges - up to Mn4+.
Manganese oxide is often used as a catch-all term for the oxides of manganese. Manganese dioxide is an inorganic compound that has the formula MnO2 and is the primary component by far of pyrolusite, itself the major ore of manganese. This blackish or brown solid material occurs naturally and is extensively mined for its colouring properties, in addition to as the primary source of manganese metal. Manganese dioxide can be used to produce manganese(iv) brick.
Hausmannite is another oxide ore of manganese, but contains manganese in both the +2 (divalent) and +3 (trivalent) forms. As such, it contains both Mn2O3 (manganese trioxide) in addition to MnO. Collectively referred to as the mixed oxide manganese(ii,iii) oxide, Mn3O4. Hausmannite is a brown-black lustrous mineral. Like manganese from other ores, it can be used to produce Mn3O4 bricks.
Mn3O4 is isolated through ferro manganese refining, and is also commonly mined conventionally. It is used as a naturally occurring Braunite/umbre in several applications. It is used to produce very dark strong brown bricks or Mn3O4 brick. It is also used to produce a dark brown brick with clays that are not stable at higher firing temperature, such clays require a stronger manganese pigment in a greater quantity to make the shade of brown required.
Among other varieties of industries, manganese oxide powder and manganese dioxide is used as a colour agent, within red firing clay to make a darker shade of slug materials, often mainly used to produce brown and dark brown ceramic bricks, facing bricks, roof tiles and pavers.
It’s used globally to make dark brown bricks and ceramic bricks that are mainly visible, such as ceramic bricks and facing bricks.
Braunite is another important manganese ore within the group of compound oxides. It'schemical formula Mn2+Mn63+SiO12, meaning that it is a silicate material with both di- and tri-valent manganese present. When in a tetragonal system it will crystallise, forming crystals of dipyramidal habit. Unlike some other ores, it is naturally formed into more compact, granular type aggregates. The colour is described as black with a semi-metallic and matte black lustre. Like the other manganese ores, braunite finds extensive use for the pigmentation of ceramics, bricks, roof tiles and pavers. For these ceramic applications, it’s very cost effective in producing natural brown hues within the fabric of the clay.
Manganese oxide is often referred to under several names, and whilst some of the compounds are chemically different, in the pigmentation space they all largely behave in the same way. Such names are: manganese dioxide, braunite, manganese umber, hausmannite and pyrolusite. For simplicity, the various manganese compounds will be referred to herein as ‘manganese oxide’ or ‘manganese umber’.
Advantages of using manganese oxide
In addition to the characteristic colouring properties, using manganese oxide brings about two other distinct advantages that improve the current brick structure, easiness to work with and overall long-term life of a brick or tile:
- it can reduce water absorption still further compared to red bricks
- improves the surface structure, thereby making it smoother and more uniform and easier to work with
Such benefits further enhance the attractiveness of utilising manganese oxides in the manufacture of ceramics, bricks and tiles.
Manganese for pigmenting bricks, tiles, and pavers
Aside from manganese metal production itself, the pigmentation of bricks, tiles, ceramics and related materials is the largest application for the mined oxides of manganese.
Manganese oxide is a synthetic compound of hausmannite, Mn2O3 (manganese trioxide) and MnO (manganosite). It's commonly used as a colour agent, or pigment for the manufacture of clay bricks.
Manganese oxide is used in the colouring and production process of bricks to produce varying shades of bricks other than the standard red bricks that are commonly seen. Colours using manganese oxide/manganese umber provide a great range of browns from dark brown to light brown throughout the body of the clay, the most common uses for manganese coloured ceramics are:
- Ceramic tiles
- Roof Tiles
- Paving Slabs
- Bricks
- Pavers
- Facing bricks
Exact colour and shading can vary depending on the type of clay used, other materials and other factors including and especially the iron content of the original clay. Typical quantities used are in the region of 1 to 8% manganese umber by mass. Bulk density of the umber is not significantly different to the original clay and thus no weight penalty will be observed.
At high loadings of umber, crystallisation will occur affording a ‘speckled’ appearance to the tile or brick. Increasing to extreme loadings - in the region of 20% by weight - a metallic-like structure is possible.
How Manganese Umber Is Applied In Producing Tiles, Bricks, And Paving Slabs
Taking the production of bricks as an example, manganese umber is added at an early stage. This is an important factor as by adding it at the earliest stage, it mixes with the clay at the first instance, and persists all the way through firing.
Shale is broken up and left to weather for up to two years. Once it has become more brittle it is then gathered and broken up into small fragments, to which water is then applied to the fragments and everything is mixed to form a cement-like, clay paste mixture. It is at this point that an accurate amount of manganese umbre is applied, calculated to the precise requirement, to create the colour of material required. Once the manganese umber is mixed in, the mixture is then extruded through a rectangular frame to produce a long rectangle piece - resembling a long brick. This is called a slug.
The slug then moves on to other parts of the brickmaking process which involve adding holes to make it lighter, cutting to the correct size, and then firing the bricks to harden off the clay. After some final product testing, the bricks are then ready. The colour has been impregnated into the material at source, so it is consistent, permanent and does not look like it is a post-manufacturing treatment.
The production of tiles and slabs follows largely a similar process, albeit the extrusion process to produce a slug is replaced in some instances by a simple molding process. Traditional production using the soft mud or dry pressing processes are also available for use with manganese umber, but this may increase unit production costs due to a greater labour requirement.
Additionally, manganese umber can be applied superficially to the outer surface of the brick or ceramic before firing, but the persistence of colouration will not last as long due to weathering revealing the standard brick colour underneath. When used as a superficial ‘coating’, the water resilience properties normally associated with adding manganese to bricks are less prevalent.
Advancements And Other Products
African Pegmatite constantly invests in resources to find new and improved ways to provide the finest pigments possible, helping to enable a designer’s vision whilst maintaining the quality, strength and durability demanded by the engineer or builder. As part of this ongoing process, African Pegmatite has developed a black brick pigment, a grey firing oxide, as well as a newly trademarked ‘true-red’ brick manganese. Not to be confused with the standard orange or terracotta, but a lustrous, rich red that bears no comparison.
K37
Developed in-house, K37 is a manganese oxide-based pigment for ceramics, bricks and related compounds to produce a variety of black colourations. A combination of oxides of iron and manganese, K37 can be added in a similar way as manganese umber to brick and/or ceramic manufacture, affording a consistent colour profile throughout.
Consultancy
When it comes to establishing ceramic pigmentation requirements, expert consultation with a trusted supplier such as African Pegmatite is recommended, partly because the pigmentation capability depends on the firing temperature stability of your clay at specific temperatures - rarely are two source clays the same. African Pegmatite’s fifty years of experience and deep knowledge can help customers in choosing the right pigment for the right clay to get the desired outcome. African Pegmatite gladly accepts sample clays sent by customers on which a full analysis will be performed and detailed recommendations provided on pigment, clay treatment, colour outcome, firing temperature and more.
Varying Products
It should be noted that adding in manganese umber at a specific point in the process during manufacture produces a much more natural finish with a consistent colour, than using other types of products that try to emulate this process after the bricks have been fired. Noteworthy, too, is that superficial treatments to bricks and ceramics are just that - any structural, weathering or workability benefits can only be realised with inclusion in the clay itself.
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