Flexible, uninsulated ducting is great for casting. The kind we’ve salvaged from construction sites a couple of times is a 24“ diameter spiral of spring steel wire wrapped in a thin, durable skin of coated fiberglass fabric.

Building a design. Normally, we’d keep the wet paper loosely attached as we find our design. However, the shapes can slide about on the curved surface, so we firmly stencil brush the shapes onto the ducting as we put them on.

The final design. The grey patches are the ducting showing through. Windows make the form a more interesting and are also useful when we’re composing & cobbling objects together to make a work.

We either create a design directly on the ducting or on a table and then transfer it to the ducting. Either way, our 8' or so of ducting starts out fully extended, straight out. We want the surface as close to a smooth cylinder as possible. We'll put a weight in either end to keep it from relaxing too much.

Usually, we'll do a single layer from sheets of double-couched abaca beaten 5 hrs and lightly (body weight) pressed.

The mirror below the mold helped us to see quickly if the wet paper was beginning to drop off the ducting as we were positioning it.

Once the design is in place and the wet paper has been lightly pressed onto the fabric, we’ll brush on a layer of methyl cellulose. After that has a few hours to soak in, we’ll twist & bend the ducting to find a position or "pose" that we like. Then, we'll brace it in place with weights, blocks, ropes & clamps. The wet paper holds onto the fiberglass fabric well if the positioning phase is gentle.

This shows how tight and dramatic the ribs can be. Sometimes, these tight bends can cause the wet paper to slide away from the fabric. Fixing that involves holding the bend apart and using a tiny stencil brush to gently push the paper back into the folds.

One could produce a tube if the paper went all the way around the ducting. Then, it could be cut off when dry. However, the form gets awkward to compose with and has a clunky catapillar look. We generally cast half way around.

For drying, we use 1/2" earth magnets on the wire ribs to restrain the paper. Occasionally, they leave a light embossment. The spring steel cause the magnets to rust onto the wet paper so to prevent that the magnets are wrapped in packing tape. When the magnets begin to go on, the ducting has to stay put. They tend to "jump" to each other if the ducting jostles much. (The rapid-fire clicking of magnets slamming together as they pop away from where you so carefully positioned them can make you weep.)

The blue tape shows how much the abaca shrank. We took this photo after 2/3 of the magnets had been removed.

The scallops between the wires and the curve of the cylinder provide the cast with a lot of restraint as well. Despite that and the magnets, the 5 hr abaca will shrink a great deal.

Depending on temperature & humidity, a conduit will dry in about a week. It's faster if there is a forced air heater blowing through the ducting.

The resulting object is durable, structural and dimensional.

common sense alert: If you try casting some ducting, use only new & hose it down well with sudsy soap to rid any fiberglass residue from the manufacturing process. Wear nitrile gloves. If you're not sure a length of ducting is safe to handle, do not use it.