The spiral triskelion shape is closed. The putty sealed the shape so when he poured powder into the tube the putty prevented the first powder from getting inside the shape. Once backfilled he removed the putty from the sealed shape and filled that empty space with a different steel powder to create the contrasting Damascus look.
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u/SeedlessPomegranate 4d ago
What was the point of the first step? The part where the putty was put on?