What Is Caulk?

Caulk is a non-structural, physical compound that provides an airtight, watertight, or non-porous seal. The seal or closure made by caulk is used to prevent something, such as air, water, gas, liquid, or steam, from entering or escaping between two other materials. Caulk is used to seal joints or spaces between those two materials whether they are similar in substance or not.

People have used caulk for centuries. It is best defined by what it does rather than by its composition. Caulk is both a material and an action; to caulk means to apply a caulking material. This article will explain both its purpose and some of its uses.

What It's Made Of

Caulk is made of materials that have been used for centuries and newer chemical substances created through the advances of modern science. You may recognize it in many forms such as the chinking used between logs on a log cabin to the rubbery substance that surrounds a shower enclosure. It is a substance that may be oil-based or petroleum-based in composition; a material made of silicone, latex, or acrylic; or, as used in older times, it may simply be tar or pitch, or oakum which had been used for a long time in nautical and plumbing (lead soldering) applications. It is applied in a gel-like or paste-like state, not in a solid, liquid, or gaseous state. Caulk itself has been refined and developed by manufacturers by using chemical combinations that allow it to be more mildew and mold, and sunlight resistant.

What It Does

The expectations of the caulk, what it does, how it reacts to materials, and the physical stress exerted are important to analyze when selecting the proper caulking medium for a particular purpose. A specific use may require that the caulk remain flexible over time, or that you be able to paint or stain it, or that it must develop into a solid mass over time. Caulk is generally pliable when applying it and, then, depending on your selection and purpose, it cures or dries to a predetermined hardness based on its physical properties.


Caulk has uses in the building trades, dentistry, medical surgery, and veterinary science (to repair horse hooves, for example), among others.

Simple Application or Caulking Process

Various applicators have been designed for ease of spreading and concentrating the material over the area(s) desired. You may be familiar with caulk and the process of caulking in bathrooms, kitchens, and around windows. A simple frame is used to exert manual or mechanical pressure on a tube filled with caulk and the caulk itself is directed to the work surfaces through a nozzle of varying size. It is as simple as using a tube of toothpaste.

Historical Background

If you ever need to explain the process of caulking to someone, you should understand the nautical application of caulk from years ago. Ships will not last on water if they develop leaks, and caulk had been used for centuries to fill the gaps between the boards the ships were made of throughout history. Where the sides of the hull and the deck met, there was one board in particular that had to be completely sealed. If the gap were to open, water could seep in from beneath the deck and sink the ship. This board was nicknamed by sailors as "the devil." Hot pitch was used on sailing vessels to seal this important gap and many times the fires keeping the pitch hot would succumb to rain, wind or seawall surge. If the pitch was not at a proper temperature, it would not adhere properly. You have heard the phrase originally used by sailors when that board needed to be caulked, "There's the devil to pay." The full phrase used at the time was, "There's the devil to pay and no pitch hot." For us today, caulk still has this kind of useful purpose.

Ronald Erich Telsch

Ronald Erich Telsch holds both Bachelor and Master of Fine Arts degrees. He is a retired juvenile justice administrator with an extensive professional writing history in that field, including national presentations for the American Counseling Association and the Virginia Juvenile Justice Association. Telsch is a volunteer firefighter/EMT and a farmer.