Almost midnight, moon is high. How can all metal plates fit together in the grid of triangles? It’s about time to conclude, to make pictures overlap. Sizes differ, rearrangements are necessary. This is just underways. Not arrived yet…

Iron/steel; watercolour and graphite on paper.

Iron, with symbol Fe and chemical number 26, is the fourth most abundant element in the earth’s crust, and by mass the most common element on Earth; melting temperature 1538°C and boiling temperature 2862°C. Iron exists in at least four different crystal structures, changing with temperature. At 770°C (the so-called Curie point) it becomes magnetic.
The reddish colour of planet Mars derives from minerals rich in iron oxide, and meteorites found on Earth can contain up to 95% iron – often mixed with nickel. Iron beads found in Egypt have been dated back to at least 3500 BCE; as the material also contains nickel, it bears witness of meteoric origin. Iron production from metallic ore only began some hundred years later. In biology, iron compounds are key components in oxygen transporting (hemoglobin), cellular respiration, oxidation and reduction.
Pure iron is soft, but can be hardened by a smelting process including carbon. The resulting alloy is called steel. Rust is the common name for iron oxide.
Iron oxides (“earth pigments”) are also among the oldest known pigments, in use since prehistoric times. There exist a range of different colours, from yellow (ochres) and red (siennas, Venetian red, caput mortuum) to brown (umbers) and black. A synthetic compound with carbon and nitrogen, known as Prussian blue, yields an intense metallic blue colour.

copper; watercolour and graphite on paper.

Copper monocrystal; photo © Theodore W. Gray

Copper: a chemical element with symbol Cu and atomic number 29; a soft, ductile and malleable reddish metal; melting point 1085°C, boiling point 2927°C; very high thermal and electrical conductivity. Together with silver and gold, copper belongs to group 11 of the periodic table. Copper and copper alloys (such as bronze and brass) have been known since at least 10 000 years. In Roman times, the most important copper ores were the ones mined in Cyprus; the Latin name cyprium – later cuprum – bears witness to this. In mythology and alchemy, copper is associated with Aphrodite/Venus. As a trace element in humans, copper is mainly found in liver, muscle and bone. Copper salts produce a range of pigments, mainly within the blue-green part of the spectrum. In  nature, copper salt pigments give colour to minerals such as azurite, malachite and turquoise.

aluminium (2nd state); watercolour and graphite on paper.

aluminium (1st state); watercolour and graphite on paper.

Aluminium disc; photo credit Theodore W. Gray.

Aluminium: a chemical element with symbol Al and atomic number 13; a silvery white, soft, ductile and malleable metal; melting point 660°C, boiling point 2470°C; does not easily ignite; nonmagnetic. Aluminium is the third most abundant of all elements in the earth’s crust (after oxygen and silicon). In spite of its common appearance, it does not seem to have any biological function.
Aluminium is, together with copper, one of the chemical constituents of the mineral turquoise. With sulfur and sodium, it produces ultramarin blue and violet pigments found in nature. It is also a constituent in potassium alum, a compound known since around 2000 BC and widely useful for its astringent, blood coagulating, flocculant, fire retarding, preserving and purifying properties. Alum is also efficacious for fixing pigments, textile dyes and making ceramic glazes. The multiple uses of alum is mirrored in antique treatises and alchemical practices. Metallic aluminium wasn’t extracted until the 18th century.
Pure aluminium can be used for metallic paints and pigments, as it keeps its silvery reflectance even in finely powdered form.

zinc; watercolour, crayon and graphite on paper.