Peep Plugs were originally designed for manual kilns, which relied on cones for temperature measurements - because all kilns came equipped with necessary spy holes for viewing one’s pyrometric cones, peep plugs (or spy plugs) were developed with the ability to be taken out and placed back in during the course of a firing. Peep holes in the kiln allowed one to “peep” inside during the firing and observe the cone positions. Modern-day kilns with digital controllers are still equipped with peep holes, which are useful for ventilating your kiln, particularly if you do not have a vent. Additionally, peep plugs can be removed at the completion of firing to help cool the kiln a little bit faster.
Peep plugs are typically made of a lightweight ceramic body that can handle thermal stress well. They are available in a variety of shapes and are manufactured by different kiln companies. However, many peep plug models are interchangeable with different kilns, and the type of plug that is chosen is typically based on individual preference. The peep plugs with a tapered shape are designed to fit in just about any hole, for example; our L&L plugs, though, are straight-sided and meant to fit perfectly in the spy holes of L&L kilns.