Ochre is the general name for a group of earth pigments, including: red ochre, yellow ochre, purple ochre, umber, and sienna. The main mineral in ochre is limonite ( iron(III) oxide-hydroxide ). This gives them all a slight yellow colour.
Red ochre takes its reddish colour from the mineral hematite (Fe2O3), which is an anhydrous iron oxide.
Yellow ochre is limonite (FeO(OH)·nH2O) and is also called gold ochre
Purple ochre is chemically identical to red ochre, but has a slightly different colour due to the way light diffracts off it due to a slightly larger average particle size.
Brown ochre, goethite (FeO(OH)), is a partially hydrated iron oxide.
Umber contains manganese, five to twenty percent, which makes them a darker brown.
Sienna contains limonite and a small quantity of manganese oxide, around 5%, making it darker than ochre.
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Although if you are sensible you should not hurt yourself.
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Step 1: Gathering the Raw Materials
As the minerals which form ochre are very common in the earth it is quite easy to find some form of ochre. However some of the better colours are harder to find as they must be a specific mix of minerals which is rarer. Many rocks contain the right minerals but due to their hardness are not ideal for turning into a pigment. because of this it is easier to collect clay which contains the minerals but is also relatively soft. The clay I gathered was from the base of a small cliff, this is a good place to find ochre rich material as there is little organic matter mixed with it.
Once gathered the clay will still have some moisture in it which makes it hard to grind, so break the clay into small lumps and leave it to dry. The smaller lumps you break it into the faster this drying will take.
Step 2: Grinding
Once the clay has dried it is ready to grind. It is best to use a hammer or similar to break the clay into many small pieces which then will fit better into a pestle and mortar. As I was just making a small amount of ochre I used a small pestle and mortar, although you can use a much bigger one or even improvise with a tough bowl and some other hard object. To begin with just smash the pieces until it is all in small grains, then use the pestle and mortar regularly until you have a very very fine powder. The finer the powder the better it will work for making paints, however, if you are going to follow the next step you may want to leave it as a coarser powder until after that step as it will thus be easier.
Step 3: Colour Changing (heat)
The ochre you have will most likely have some natural sienna and umber pigments in them. When these are heated, they are dehydrated, transforming some of the limonite into hematite, this makes them more reddish in colour, and the resulting pigments are called burnt sienna and burnt umber.
To heat it you will need to place the ochre in a heat proof container, I used a copper bowl (it is silver due to a patina), then spread it out to increase the surface area and heat with a blow torch, being careful not to blow the powder away, this is why it is better to finish the grinding after this step. You should do this outside as the powder could contain minerals which when heated will decompose and could give off some toxic gasses. As you heat it you should notice some colour change and can stop heating when the desired colour is reached. I would advise you test this on a small amount of your ochre to see if the colour change is desirable. You still may not find the colour of the ochre is quite as you want it but don't worry as you can change the colour in other ways.
Step 4: Colour Changing (additives)
A simple way to improve the colour is to add in another pigment. This can be especially useful when you are trying to make a colour like red as ochre is much easier and cheaper to get than some other red pigments, however, ochre is not the most vivid of colours so adding a small amount of harder to get but more vivid colours can produce a nice compromise. As you can see from the picture I am adding red to give my reddish brown ochre more of a red hue. The pigment I am adding is Dragons Blood which I have talked about in a previous instructable. To add the pigment grind it well and then add it to the ochre in a pestle and mortar and grind more to ensure a good mix. You can alter the colour by varying the amount of additives you add. You could also try making the pigment darker by adding Lampblack or making a purple by mixing red or purple ochre with Verdigris. If you find different clays you could also mix different ochres to achieve the exact hue you desire.