Preparing the fibres:
Before dyeing fibres, whether it is for natural or synthetic dyes, we will need to scour them. This means, we need to clean them properly so that there are no residues of grease, dirt or chemicals left. If there is dirt left in the fibres, the dyes will stick to the dirt and will disappear with washing them afterwards or give your hands a nice, but often undesirable colour.
For raw uncarded wool straight from the fleece, I soak it for several hours and keep repeating it as long as a lot of dirt comes out (note: you can use this rinsing water for your plants, it is rich in nitrogen). Now add dishwashing liquid to the water and let it soak for a while. You might have to do that a few times and rinse it well afterwards, until the water becomes clean. Be wary to never pouring hot water onto the fibres or handle them roughly , because the wool might start to felt.
Wool slivers are cleaned, washed and carded, so there is no real need to go through this whole process. You will still need to soak them in clean water before mordanting or dyeing. The only risk with the slivers is that they can have residues of chemicals in them, which might interfere with the dyeing. This is one of the many reasons to use organic wool slivers, that have been processed without chemicals.
Silk fibres or fabrics need to be gradually brought to just under the boiling temperature and kept at that temperature for 1 hour. Boiling can damage the fibres. If the silk has not be de-gummed yet, it needs to be heated up with dishwashing liquid. Repeat this process until the silk doesn't feel sticky anymore.
Vegetable fibres, such as cotton, hemp and linen need to be boiled off with washing soda for 1 hour. If the fibres are new and still have wax in them, you will see the water turn yellow to light brown. In that case it could be safe to repeat the process. If you need to prepare a lot of fabric, you can put it in the washing machine at the highest temperature, close to the boiling temperature and add the washing soda to it.
When the fibres are well clean, they can be used for dyeing.