Knife in the Dark

by Eric Hart

Cheryl Koski in Triad Stage’s version of Wait Until Dark
Cheryl Koski in Triad Stage’s version of Wait Until Dark
The best solution for a throwing knife at Triad Stage was also the oldest

The stage is dark but for a single light. The heroine is at the fuse box, ready to pull the fuse and plunge the apartment into darkness. The villain throws a knife at her, and it sticks in the wall just above her head.

Having a projectile hit a wall is a tricky problem in theatre. Numerous plays call for arrows shot into walls, or, as in Wait Until Dark, a knife. The safest and most consistent solution is for the actor to pretend to throw the knife, while a duplicate knife pops out of the wall. With precise blocking and timing, the audience believes the duplicate was thrown by the actor.

I asked special effects wizard Seán McArdle for any high-tech solutions to this problem. “Solenoids aren’t fast or powerful enough. Pneumatics make too much noise,” he told me. The most reliable method was also the oldest: a rubber band.

I started by building a small box with a “track” that attached to the back of the flat where the knife would appear. I then cut a small “sled” that rode in the track. On the sled, I attached a duplicate of the knife handle. A rubber band would be pulled taut when the sled was all the way at the back of the track, and some kind of mechanism would hold the sled in this position. When the mechanism was released, the rubber band would spring the sled to the front of the track, and the fake knife handle would extend out of the wall.

 

The sled fully retracted in the track, at left, and the sled when extended, at right. This whole unit (plus the surgical tubing) was mounted to the back of the flat, where a hole was cut just large enough for the fake knife handle to fit through.
The sled fully retracted in the track, at left, and the sled when extended, at right. This whole unit (plus the surgical tubing) was mounted to the back of the flat, where a hole was cut just large enough for the fake knife handle to fit through.

I used MDF for the construction of the track and sled to give myself a smooth and flat surface with little friction. I used wide, flat pieces so nothing would bind or get stuck. An ample coating of paste wax helped ensure smooth movement throughout the run of the show.

 

 

Here the unit is mounted, and the loop of surgical tubing is added. When the mending plate on the right pivots out of the notch, the sled is free to slide forward until it hits the flat.
Here the unit is mounted, and the loop of surgical tubing is added. When the mending plate on the right pivots out of the notch, the sled is free to slide forward until it hits the flat.

My prototype used a rubber band to propel the knife. That proved too weak and slow for a convincing effect, so I switched to a loop of surgical tubing (sold as “latex hose” in home improvement stores). One end was attached to the back of the flat, while the other to the back of the sled. Even when the knife was fully extended, the loop remained a bit taut; this minimized any “bouncing” of the knife back into the wall, which might ruin the illusion that it was being thrown from the front.

 

 

A view from the front of the wall with the knife extended. Every night, a bit of adhesive-backed paper painted to match the wall is used to cover the hole until the knife bursts through.
A view from the front of the wall with the knife extended. Every night, a bit of adhesive-backed paper painted to match the wall is used to cover the hole until the knife bursts through.

I attached a short mending plate that pivoted freely near the track. I cut a notch in the sled; when the plate rotated into the notch, it held the sled in place. When the plate rotated out of the notch, the sled became free to spring forward. With a string tied to the other end of the plate, releasing the knife became as simple as pulling the string downward. Ease of operation was essential, since the backstage crew needed to activate this in the dark.

 

To reset the trick, the crew would simply pull the sled back and flip the plate back into the notch.