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What are pipe-cleaning brush stems and cores made of?

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The stem and core of a pipe cleaning brush (AKA tube, spiral, bottle brush) are usually made from galvanised steel. The stem and core is almost always made from this material:

Galvanised Steel

Galvanised steel is used for many things other than pipw cleaning brush bristles, such as this bucket.  Galvanised steel has been treated to avoid rusting. It is coated with zinc, which binds to the steel creating a non-rusting layer. The zinc layer is also more durable so it doesn’t get scratched so easily and looks new for longer.
galvanised steel wire can be bent without breaking making it suitable to twist  into the stem of a pipe cleaning brush AKA tube brush, twisted brush, spiral brush The galvanised steel wire can be bent without breaking so it can be twisted together to form the stem and core of a pipe-cleaning brush.