Brewarrina Ochre Beds
Just downstream of Brewarrina are ochre beds in the river banks. Aboriginal people mixed the ochre with water to make a paste for art work, including body art for traditional ceremonies. The beds are protected under NSW State Heritage listing.
Earth pigments
Ochre is a family of earth pigments, which includes yellow ochre, red ochre, purple ochre, sienna, and umber. The major ingredient of all the ochres is iron(lll) oxide-hydroxide, known as limonite, which gives them a yellow colour.
Yellow ochre (Goldochre)- pigment
- Yellow ochre, FeO(OH)·nH 2O, is a hydrated iron hydroxide (limonite) also called gold ochre.
- Red ochre, Fe2O3, takes its reddish colour from the mineral hematite, which is an anhydrous iron oxide.
- Purple ochre is identical to red ochre chemically but of a different hue caused by different light diffraction properties associated with a greater average particle size.
- Brown ochre, also FeO(OH), (geothite), is a partly hydrated iron oxide.
- Sienna contains both limonite and a small amount of manganese oxide (less than five percent), which makes it darker than ochre.
- Umber pigments contain a larger proportion of manganese (five to twenty percent), which makes them a dark brown.
When natural sienna and umber pigments are heated, they are dehydrated and some of the limonite is transformed into hematite, giving them more reddish colours, called burnt sienna and burnt umber. Ochres are non toxic and can be used to make an oil paint that dries quickly and covers surfaces thoroughly. Modern ochre pigments often are made using synthetic iron oxide.
Ochre contains a minimum of 12% iron oxyhydroxide, but the amount can range up to 30% or more, giving rise to the wide range of colors from light yellow to red and brown. The intensity of color depends on the degree of oxidation and hydration of the iron oxides, and the color becomes browner depending on the percentage of manganese dioxide, and redder based on the percentage of hematite.
Since ochre is sensitive to oxidation and hydration, the yellow can be turned red by heating goethite (FeOOH) bearing pigments in yellow earth and converting some of it to hematite. Exposing yellow goethite to temperatures above 300 degrees celcius will gradually dehydrate the mineral, converting it first to orange-yellow and then red as hematite is produced. Evidence of heat-treatment of ochre dates at least as early as the Middle Stone Age.
Ref: https://en.wikipedia.org/wiki/Ochre
https://www.thoughtco.com/ochre-the-oldest-known-natural-pigment-172032
To be able to log this cache as a find you must send the answers to the following questions to the CO.
1. What is ochre?
2. What mineral produces different colours in ochre?
3. What are the predominant ochre colours at GZ?
4. What is the colour of the ochre underneath the natural overhang at GZ and estimate the percentage of iron hydroxide in the ochre here.
You may log this Earthcache straight away but then please email your answers to the questions to the CO. Logs with no answers sent will be deleted. Please do not post your answers in your log.
A photos of you or your team name in the general area at GZ must be posted with your Found It log, but no spoiler photos please.
While you are in the area you will want to visit the nearby fish traps that you can see just upstream, which are claimed to be the oldest man made structures in Australia.