So, the summer is here, the sun is out, it's the perfect opportunity for you to try
some photochemistry with your classes. Behind my trusty mini whiteboard here,
I have a piece of paper, on which we've painted some cyanotype mixture and in
front of that, I've placed an acetate with an image of the structure of
Prussian blue – the compound that we're going to make. Let's see what happens
when I remove the mini whiteboard and expose it to the sun over a 10 minute period.
As you can see, over the course of the last few minutes we've gone from a green to a blue colour.
What we need to do next is take this indoors
rinse off any unreacted dye and make sure that we're left only with the
Prussian blue fixed to the paper. Then I'll show you how I made up the mixture
and applied it in order to produce this really cool, simple little demonstration.
So, I found some slightly dimmer conditions to talk about the actual chemistry
and how you go about making some of this paper and then we can go in
and see if we can develop the print that we just made.
So, the Herschel cyanotype mixture basically requires a mixture of citrate and iron(III) ions
in the presence of some potassium hexacyano ferrate(III). So, what I have here is
20 ml of water, into which I've dissolved 0.6 g of citric acid and
0.6 g of iron(III) chloride-hexahydrate it's important to use the hydrated form of this.
The citric acid we're then going to neutralise with some ammonium carbonate
and at this small scale, you can actually go and chuck in a decent amount, all together.
If you're following a CLEAPSS recipe
they do recommend doing this in small sections, but we're using only a
very small quantity of liquid and so the foaming is not too bad. And now we've got
some ammonium citrate in there with our iron(III) chloride, all we need to do is add
1 g of potassium hexacyanoferrate(III) and
we're good to go. That will produce a greenish-coloured
solution and that you can paint straight on to some paper. I find the magic
whiteboards the little erasers that come with them
those sponges, they're pretty good for painting this straight onto a
piece of paper. And you might want a hairdryer to help you to speed up
getting that paper nice and dry. In dim conditions, this stuff is going to be
pretty stable – it will hang around for a decent length of time in the dark, but
obviously you want to be keeping it away from direct sunlight until you're ready
to actually do the experiment. Now that we've got an idea about what's
going on here, let's go and see if we can develop the print that we took earlier
and all I'm going to need to do there is rinse off the unreacted citrate, iron(III) chloride etc.
because what takes place is that in the presence of the sunlight the
iron(III) from the iron(III) chloride will be reduced whilst the citrate is oxidised
and then we get our mixed oxidation complex – the Prussian blue,
which precipitates out essentially as a solid, meaning that the rest of the
soluble materials can just be rinsed away in some water and we should be left
with a nice blue permanent print of whatever we were actually taking an image of.
So let's go and rinse that off and take a look at the product.
Once your exposure is complete you want to be able to rinse off any unreacted starting
materials. I actually expected to leave this out for about 10 minutes, in the end
it actually only needed about 3 minutes, because up until then I'd been working indoors.
It's surprising actually how much of an effect the the drop-off from the UV can
be coming through some glass, so do be aware of that. But you can eyeball it and
you can see that I was able to get a reasonably accurate exposure of my
structure of Prussian blue here. So what we essentially want to do is just leave
that rinsing for a couple of minutes – I find that a Gratnell tray leaning in
the sink like this so it can flow through the handle area
works quite well. And we just want to make sure that we
thoroughly washed away any of the unreacted starting material.
The image after that will be fairly stable for the long run.
And there we go – a nice permanent recording of our structure
of Prussian blue here, taken in cyanotype.
So, now we've covered the basics of getting a nice print,
let me share with you a few of my mistakes that I've done in the past,
or some other ideas that might help you get the best results with your class.
Now, the first thing you want to make sure of is that, before you do a print,
the paper is as dry as possible. If you can't get the paper dry – let's
say for example you want to do everything within one lesson – then I
might suggest rather than putting an acetate over the top that you want to
actually go for something where you're laying things on the ground – so you can see
I have a print here on the top right where I've laid some pencils and some
rulers and we've got some protractors and they're showing up quite well –
that's something that you can achieve reasonably well.
If you go for this object here up on the top left, what I essentially did was
I collected some leaves and some grass, but you can see that before I did the final
rinsing step, I didn't properly wait for it to dry and so a lot of the
blue colour rinsed out – it wasn't firmly set and firmly precipitated and bound
with the the paper, so you will lose a lot of the blue colour if you try
and rinse off too quickly. If you really want to see the colour change in one
lesson, potentially you can then protect the paper in the dark, leave it
to dry out before doing the final rinse and revealing the end firm fixed product
to your class the next time. Similarly, you can see down on the bottom
left here, I tried to get an acetate result here with a piece of coral and
you'll notice that there's some striations here – essentially this is what
was happening where it wasn't able to fully dry out from underneath the
acetate sheet and so we've got these markings here associated with
where the acetate was allowing it to dry because
there was a little bit of throughput of air, or not. Similarly, down on the bottom
right-hand corner, again I didn't really let this one fully set.
So, in terms of tips to make sure you're getting the best possible results from
this experiment: make sure your paper is as dry as possible before you do it.
If you can't wait, make sure you leave it on a flat surface and put objects
down on top of it, that are going to allow it to dry around them. And then if
it's not fully dry, make sure that you leave some time for it to dry before
you do the rinse off. Other than that, this is a pretty bomb-proof experiment.
The other thing to make sure of, is that you really do need some strong sunlight
for this to work in a sensible timeframe, so have a think about that and make sure
that you time your experiment appropriately. It's such a joy to
actually experiment and try and work out what kind of images you can get from
this, so give it a go and perhaps share some of the images you collect with us,
down in the comments below. See you next time!