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When detective fiction first began to be become really popular and mainstream around the 1920s and 30s there were a few ways that criminals could cover their tracks. Hairs left at the scene? No problem. There was no DNA testing then as there is today. Needed to destroy fingerprints on a knife? No problem, just wipe it clean (good back then but not so effective anymore as colour-changing fluorescent films can help to capture latent or hidden prints). And if there was blood on your clothing? Simple; give it a darn good wash.
But given the improvements in police forensics today, would putting your blood-stained shirt through the washing machine be enough to destroy the evidence? You might think that if you use a really, really good laundry liquid and your shirt comes out spot-free then that would be enough. Use a top-notch bleach and you’ll be in the clear. But will you?
The answer is both yes…and no…
It all depends on what kind of bleach you use. There are two types of bleach; chlorine based bleaches and what are known as oxygen beaches. Chlorine bleaches can clean bloodstains so that they will no longer be visible to the eye, but they won’t stand up to deeper forensic investigation. If luminol or phenolphthalein is applied to the clothing then, even after multiple washes, if blood haemoglobin is present it will become very clear. The luminol is sprayed onto the fabric and will glow its trademark luminescent blue (hence the name) if a catalyst is present. This catalyst is the haemoglobin in blood.
However, if oxygen bleaches (bleaches containing an oxidising agent such as hydrogen peroxide) are used then it’s another matter. In these cases, any haemoglobin is totally removed and can’t be detected. This is a bit of a headache for investigators as any lingering stain which might happen to remain on the fabric which is being looked at can’t be definitively identified as being blood, so the suspect can claim that it came from a totally innocent source. Forensic experts are currently researching ways to overcome this problem.
So next time you’re reading a book in which the criminal washes the blood out of their clothes and breathes a sigh of relief remember; they’re not necessarily in the clear. Unless of course they tell you that they’ve used an oxidizing bleach…
(N.B. We are by no means advocating buying some oxygen bleach and going out on a killing spree! This is purely an interest article for fans of speculative crime fiction.)