Biros, felt tips, marker pens.. a pen is an everyday item, found in a myriad of colours and for all uses. But what if I told you that black ink isn’t just black; or brown isn’t brown.
In many pens the ink used is a mixture of chemicals that are brought together to achieve the desired colour and consistency, relying on the use of primary colours (blue, green, red) to generate all the others.
We can study the contents of different inks using a technique called chromatography, literally “drawing a colour graph”. Chromatography is the science of separation and is used in the chemical industry in many forms from purifying chemicals in a reaction, studying the contents of water, to testing for the presence of illegal compounds in athletes.
Compounds in a mixture will interact with a liquid and an absorbent material in different ways, allowing for separation to occur. In the same way that scientists can separate and study 100’s of chemicals in the human body we can turn the black ink into its components.
Why don’t you give it a go for yourself?
For this you will need:
- a coffee filter paper (ideally, but a sheet of plain kitchen roll or heavy white tissue will work too)
- one or more black felt-tip pens with washable (water-soluble) ink (black works best, but you can try other colours too)
- a tall glass jam jar or tumbler (10–15 cm tall is ideal)
- a stapler or a paper clip
- a thin pencil (or stick of similar thickness)
Remember to do this in the kitchen!
Cut a strip of the coffee filter, or similar paper, a little longer than the height of your jar and about 20 mm wide. Draw a pencil line across the strip about 10 mm from one end of the paper. Fold over the other end, and staple it into a loop (or hold it in a loop with the paper clip), then put the pencil through the loop. Draw a small blob of ink with the felt-tip pen on the pencil line. It’s best to go over this blob several times to get as much ink on it as you can, but don’t let it get more than about 2 mm in diameter. Rest the pencil across the top of your jar, so that the paper hangs down inside it. The paper should not be touching the bottom or the sides of the jar.
Very carefully add water to the jar until it just touches the bottom of the paper (Figure 1a). It must not come up as far as the ink blob. Watch and wait. You should see the water rising up the paper, bringing the ink with it. After a few minutes, you should begin to see different colours appearing in the moving water front, as the different pigments in the ink separate out. Take the paper out when the water front is at least 1–2 cm below the pencil, since the water front keeps on moving for a while after you have removed it and it shouldn’t be allowed to reach the pencil.
Mark the maximum distance travelled by the water and then leave the paper to dry. You might like to repeat this to try the effect of different-sized spots of ink (include a sample with a very large spot) and with different makes of pens and also mixtures of different coloured pens (Figure 1b) to see if you can separate out the original colours again.
We can see from these pictures that different inks have different components. Black ink isn’t just black!