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Craftsmen of Yore (職人達の昔日 Shokunin-tachi no Sekijitsu?) is the 4th chapter of the 10th volume and the 36th chapter overall of the Attack on Titan: Before the Fall manga, written by Ryō Suzukaze and illustrated by Satoshi Shiki.
Angel recalls how he first came up with the design of his device, thanks to his assistant and fellow craftsman.
After working as a craftsman for three years, Angel Aaltonen has gained enough recognition to be given the nickname "King of Inventors." He arrives back to his workshop dejected by the results of the most recent expedition outside the Walls. His assistant Corina assumes that his friend Sorum died, but Angel laughs it off; he becomes solemn and tells Corina that the Titans were lobbing the heads of the eaten soldiers over the top of the Wall. Corina claims she is glad that she did not see it, but Angel viewed it as a reminder that Titans are a very real and dangerous threat; he then expresses disappointment that he has no information if the numerous inventions he supplied to the Military work or not. Angel wishes that himself, as well as the military, could be able to study the Titans to find a possible means to fight against them. He then spots a reed of bamboo propped against one of his furnaces. Corina explains that someone from the Corps brought it to them as a means to possibly incorporate it into one of their inventions. Angel shows amazement when he feels how cold it is and the bamboo's durability. This discovery renews his hope that there may be a means to defeat the Titans.
Within a few days, Angel and Corina are given one week's leave to go to the newly built Industrial City. Accompanying them is Angel's friend Sorum as well as master foreman Xenophon Harkimo. Despite his unease around the older man, Angel and Xenophon discuss possible ways to use the iron bamboo and both grow to respect the other. After arriving at the city, Sorum takes Angel and Corina to an underground caldera full of water; it is then revealed that the "water" is actually frozen gas that Sorum calls iceburst stone. As Angel studies a sample later that night, Corina and Angel continue to pour over ideas on how to use both the bamboo and iceburst stone. Recalling what Survey Corp soldiers have told him, Angel theorizes that the main difference between humans and Titans are the size difference; Corina proposes an invention that could eliminate the height difference. The two then spend the rest of the week developing a prototype design of an invention that allows vertical movement. Angel gives a demonstration and is surprised by the power and speed of it; nonetheless, Xenophon is inspired by it and proposes the usage of iron bamboo for a weapon to go along with it. Angel notes how Corina showed vast improvement during their trip and Xenophon showed interest in working on improving the design. Invigorated by this success, Angel states to himself that he looks forward to continuing to work with both for years to come.