The spooky chemistry behind your Halloween fake blood

Halloween would not be the same without a chemist or two.

It is the night when the veil between the living and the dead is said to be at its thinnest. Around 2,000 years ago, the Celts, including those right here in Scotland, celebrated Samhain to mark the end of the harvest. Bonfires were lit and masks worn to confuse wandering spirits believed to slip through from the ‘other side.’ Later, the Christians had their go, renaming 1 November as All Saints’ Day, with All Hallows’ Eve on 31 October and from there, Halloween was born.

Scottish and Irish immigrants then brought the holiday to the US, where it transformed into the festival we know today. But behind the scenes of all the gore and glitter lies an often unsung hero: chemistry.

Whether you are dressing up for a night out in St Andrews, heading to the Union for fright night, or filming a horror short on the West Sands, it is chemistry that brings your zombie to life quite literally.

Back in Celtic times, people used natural materials like red ochre, charcoal, malachite, and plant extracts, creating face paints for ritual disguise. That might have been primitive chemistry, but chemistry all the same.

By the early 1900s, makeup had turned into an art form. Thanks to advances in cosmetic chemistry, today's Halloween makeup is safer, longer-lasting, and far more dramatic.

Aside from colour, your face paint likely also uses an emulsion, a water and oil mix that prevents your creepy clown face look from budging all night long.

No Halloween in St Andrews (or anywhere else), however, is complete without fake blood or ghastly wounds, and again, chemistry takes centre stage.

Realistic blood needs to flow, stick, and look like real blood. Chemists found that corn syrup or glycerin mimics blood’s viscosity. Add food-safe red dye, and you have got yourself a horror movie in a bottle. The perfect colour for fake blood comes from red 40, a water-soluble synthetic dye that has also unfortunately made its way into many American food products. A recipe which reflects all of the desired characteristics is ⅓ cup of water to 5tbsp of cornstarch, finishing by adding a few drops of red food colouring.

On its own, Red 40 is too bright, so to make it more blood-like under harsh lighting, chemists add a hint of blue or green to darken the shade. That is colour theory in action.

Want to look like a desiccated vampire or peeling zombie? Then you will need latex. Latex is a colloidal suspension of natural rubber in water. Once applied, it dries to form a thin, flexible film that moves like skin, ideal for burns, blisters, rotting flesh, or axe-in-the-head effects. For those of you amongst us who are allergic to latex, synthetic options like neoprene exist, though they might be slightly less skin-like. So, whether you are heading to a party in Sallies Quad or filming a short horror on Lade Braes, latex is your best friend for believable gore.

Want to truly impress your fellow students?  Use thermochromic pigments, that is, materials that change colour and form with heat. Apply them to a fake wound, and as you heat up from dancing in the Union, your wound will “bleed” or change colour in real-time.

So, while the Celts may have started Halloween here in ancient Scotland, modern chemists have taken it to a whole new level of horror and artistry.  From the eerie glow of your paint to the fake blood dripping from your axe wound, every spooky detail owes something to chemistry.

This year, whether you are wandering the cloisters in costume, summoning spirits on West Sands, or simply layering on fake blood for a night out, remember: behind the magic, it is molecules doing the work.

Happy Halloween, St Andrews, we will see you in the shadows on the 31st.

Illustration by Mia Fish