[thedoc (OP)]

Dear Chris
Hi it's Grace from Waterbeach I'm am 11 and I have a question - why do blue biros smell salty?
Asked by Grace Higgins


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[chapter podcast=1000009 track=12.11.18/Naked_Scientists_Show_12.11.18_1000113.mp3]  ...or Listen to the Answer[/chapter] or [download as MP3]

[thedoc (OP)]

We answered this question on the show...

Chris -   I don’t think they necessarily smell salty, but I know what she means because if I scribble with my pen and my bit of paper, there is a distinctive smell.
Biros are actually quite clever.  They're really a feat of amazing engineering.  They consist of a little ball in the end of the biro and that is rolling around freely and connected to that straw-like structure inside the pen which has got ink in it.  The ink has just the right viscosity or thickness so that when the ball rolls around, a thin layer is plastered over the surface of the ball.  The ball goes around a bit further, transfers or prints that ink onto the page, leaving a line and then rolls around a bit further and picks up more ink.  Now to get that ink to behave in that way, you’ve got to have just the right runniness and then be very quick drying.  They do that using chemicals which are called solvents and one of them is called polyethylene glycol.  It’s the same stuff that you put in anti-freeze and it makes things runny, but it evaporates very quickly.  It has quite a strong sweet odour.  So, there are a number of these solvents, one of them being this anti-freeze-like molecule.  
So I suspect what Grace is smelling is one of the solvents as she writes with her pen.  And lots of pens use a similar thing.  If you use a felt tip, they haven't got a ball in them, but they’ve still got ink which has got solvents.  So things that dissolve ink in with molecules and when you draw a line on the page then the solvent evaporates, leaving behind just the ink molecules stuck into the page surface.
Dominic -   Is it safe to be smelling that solvent?
Chris -   Well, in the concentration you'll get from a Biro, I don’t think there's too much risk.  It’s more dangerous what you write with the pen than what you'll find actually contained in the pen.