Optical brighteners

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Today, a fortunate accident led me to Tree Marie's video of transparent ombré. She is unhappy about the tinge of yellow in her final M&P base. What she does to relieve this is that she adds few drops of purple dye (soluble). Genius idea to compensate the yellow hue, and to shift the visible impression to a neutral colour!

On the other hand, the yellow to start with was a “blue light missing” situation (increased absorption by phytochemicals from the coconut oil and slight caramelisation of the sugar). Addition of a blue-violet soluble dye “neutralises” this by removing more light, but from the yellow parts of the spectrum – in the end the colour is neutral, but technically grey, rather than white.

BUT the poison cabinet of modern chemical industry has another approach up its sleeve: optical brighteners. Best known from laundry detergents and paper. These are fluorescent dyes, colourless under visible light, but once they are illuminated with UV light, they convert it into blue light, and add this to the visual impression (“whiter than white” effect), rather than remove it. A “true” white at full brightness (if dosed correctly, and under the right lighting conditions).

Now, a yellowish off-hue isn't restricted to dirty laundry and half-bleached paper – but also most oils for soapmaking bring some yellow colour to some degree. Among all the clever ideas that people came up with to fix things I had never thought about, I am not aware of adding optical brighteners to “undo” the inevitable yellowness of soap. They wouldn't be the craziest chemical additives to alter the visual appearance of soap.

Not that I cared about a neutral white soap, lol. Titanium dioxide & friends (white pigments) already do a terrific job to do so. If there is a reason to have a white soap at all. Why put extra effort (and extra chemicals) into a product that will only be seen, for most of its life time, under questionable bathroom lighting anyway? For whom does the presence/absence of that off-colour matter, and why? It's a natural product made from self-conscious ingredients, which are allowed to self-consciously show off their colour, lol!
(I added this paragraph just to emphasise that I see this as a purely academic topic, and doesn't address any actual problem.)
 
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Well, I see. Guess who is in charge to try it out 🙄

I searched around how to get hold of reasonably pure optical brightener. Turns out that it's not very difficult: drugstores sell tissues soaked with these dyes, made to wash them along your white laundry to boost their brightness (no idea how they're called in English):
ob_laundry_tissue.jpg
They originally have that greenish yellow colour. I cut out that tiny 1cm×2cm piece on the bottom right, and soaked it in rubbing alcohol. The OB dye dissolved easily, the alcohol stayed clear and colourless to the naked eye, but started to fluoresce bright blue under UV light (think of tonic water). So far so good.

How to get this into soap now? I took the lazy shortcut and grabbed some M&P base that has some yellowing from the canola oil and calendula-infused jojoba in it. Unlike CP soap, M&P is readily compatible with ethanol, so I didn't have to care too much.
I prepared two batches, one with the OB solution, one with the same amount of pure alcohol. Melted up in the microwave, cast one soap bar of each, let them set at a cool place.

And, folks, it works!
daylight.jpg
It's not a mind-blowing effect, but decent enough for a first try. Left is the control bar, right is the one holding the OB infusion. This photo is shot under daylight. Viewed from directly above, the top actually looks mostly colour neutral, unlike the definitely yellow control bar.
As expected, things become less spectacular under artificial lighting (LEDs with very low UV emission):
led.jpg
But no worries, there's a trick to find out which is which: my disco light aka the good ol' UV tube:
uv.jpg
Ohhh, how my camera HATES these violet and fluorescent colours! I eventually gave up fiddling around to get the hues anywhere near the visual impression. One last thing to show (but as a B/W photo). I washed my hands with the OB soap, rinsed it off properly, and then went to the UV light again:
uv_hand.jpg
The dye sticks to skin! In fact, the UV light even made visible where I had rubbed the piece of OB tissue over my skin, several hand washs ago! It is kind of scary that you can see so many things under UV that you actually think are inexistent or at least invisible (related XKCD comic). And it is scary to directly see (not only believe what people with microscopes tell you) that you cannot rely even on vigorous and multiple washing to get rid of all the stuff that is sitting on your skin.

With the original use case for the OB tissues, this somehow makes sense. They use an OB that attaches strongly to the things it comes in contact with. You want the dyes to stick to the laundry, rather than just being rinsed away into the sewage.

Lesson: if one adds OBs to soap with serious intentions, watch out for an OB dye that is not specifically made for sticking to everywhere.
 

Dawni

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Silly question. Did that soap "brighten" your hands too?

Maybe you've discovered the secret to those whitening soaps 😋
 

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