Spoiler Alert: Heavy spoilers ahead for the entire season of The Undoing.
It's unfortunate but, after watching the recent finale of HBO's The Undoing, my Google search history officially looks like that of a deranged killer. No regrets, though, because I desperately need to know what happens to all the blood and fingerprints when you put a hammer in the dishwasher. Could a regular dishwasher — maybe on the pots and pans cycle? — rid a murderous hammer of evidence to the point that it would be deemed unusable in a court of law? Basically, I was wondering: Did The Undoing's Henry make a smart — albeit very illegal — move? Or was it simply a rash pre-teen, scared-as-hell move? Or, both?
The answer to all three of those questions is probably: yes. When Henry finds the hammer-in-question wrapped in paper in an outdoor fireplace, we can see that there is some dried blood on its metal head, as well as some blood splatter down the wooden handle. He then runs it through the dishwasher twice (questionable how he did this without anybody noticing, but I guess when you're The Undoing-level of rich you have more than one dishwasher?), and stows it in his violin case. When Henry's mom finds the hammer, it's completely clean. Like, so clean. Like, all the blood and — we assume — fingerprints are gone levels of clean. It's the kind of clean that had me wondering: Is this even possible?
I'm not a hammer-cleaning expert, but I do own a lot of metal pans and wooden spoons, and so I know that if you put them in the dishwasher, you can definitely get a lot of stuff off them — including oil. Since fingerprints are just oily residue, the guilty party's fingerprints certainly could have been at least partially washed away. Blood, though, is a different matter.
According to Murphy Oil Soaps, to remove blood that's sunken into wood, you'd need steel wool and bleach, which would leave a bleach stain, meaning the hammer handle would have been stained lighter — and not in a uniform way, making it pretty evident that it had been tampered with. Plus, the blood would have had time to sink into the wood since it spent 24-plus hours hidden in the fireplace prior to being washed. While the blood in question probably only belonged to the murder victim here, there could have been traces of the murderer's blood as well.
There's another risk involved with cleaning a murder weapon in a dishwasher though: Trace DNA could be left inside the dishwasher, too. So even if the hammer provides no evidence, the dishwasher could.
Because my curiosity has no end, I also spoke with a crime tech (who wished to remain anonymous), who confirmed my suspicions. "Due to the high evidential value of this item, it would always be subject to high-end forensic examination," he said. "So, though [the dishwashing] would hamper recovery, under the right conditions it could yield both DNA and or fingermarks." While the actual findings would depend on quite a few details (how hot the water was, type of detergent, etc), he assured me that, although a dishwasher could "potentially destroy any useful evidence," some items have survived very harsh conditions and still yielded evidence years after they were used in crimes.
So, did Henry actually do a good job at protecting the murderer (in this case, his father, yikes) by running the hammer through the dishwasher (twice!)? Honestly, probably. At the very least he would have gotten some of the fingerprints off and probably some of the blood, too — especially the blood on the hammer's head.
As to the question of whether or not he should have obstructed justice in the first place? Well, he's just a kid who wants a dog and a functional set of married parents, so can you blame him?