Campus Tours:Cob Overview
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What is it?
Cob, is an earthen plaster made by mixing sand, clay and straw. The materials are available locally resulting in easy accessibility, fewer transportation costs, and fewer pollutants. The straw is a “quick renewable” in that it’s growing time is very rapid in comparison to other building materials such as trees. It has many uses such as coating walls, making sculptures, building staircases, constructing handy household items and more. It adheres well to the straw bales making them virtually fireproof and giving the walls a finished look. The hard crust that is formed upon drying prevents pests from burrowing into the walls and making themselves at home. The cob can be colored with paints on the surface or pigment mixed into it depending on the purpose and preference of the artist. Sometimes shredded newspaper is added to improve the workability of the mixture.
You will see cob incorporated into nearly every aspect of Old Main from design and decoration to highly functional art such as the cob staircase and wiring trough.
The walls can breathe!
The health of the straw bales is critical to the lifecycle of the building. The cob walls allow the passage of moisture into and out of the straw bales. When coloring the walls it is important that only breathable paints or pigments be used to allow the continued passage of moisture. If a sealing paint is used the moisture will not be able to escape and will condense in the bale, rotting the walls from the inside out. The walls themselves act as a moisture buffer by absorbing excess moisture in the air during spring and summer and releasing moisture when the air is extremely dry in fall and winter.
We have been applying cob in two to four layers depending on the irregularities in the walls and the desired final texture and design. The first layer is a runny “slurry” layer (clay slip) to coat the straw bales. Additional layers then smooth out, fill in and finish the wall. In some situations wire mesh is used as framework for the cob: to add strength, serve as an armature under a sculpture or as a material for the cob to adhere to.
Fun & Functional
Cob is amazingly versatile and can be used just about anywhere. There are countless examples around Campus of the things cob can do and the artistic tendencies it brings out in those that dare to get their hands dirty. It can be used to finish walls, form shelves, staircases, garbage cans, coat racks, paper towel holders, decorative bowls, placques, end tables and more. The cob can be water-proofed using polymers or beeswax and peanut oil.
Durable
- In Devon county in England, 20,000 cob houses are still in service after 500 years of use.
- The oldest sections of the great wall of China are made of cob.
The cob staircase and section of cob in the first floor of Old Main have allowed us to see just how durable cob is. The staircase is subject to daily traffic as is the section of floor. To date, both are holding up quite well.
Labor Intensive
Mixing and applying cob can be very labor intensive. We are researching methods to make these processes quicker and easier than they are today.
Stages of cob
Cob recipes
Cob is pretty forgiving, but the strongest mix is about 70% sharp sand and 30% clay. Add straw to your liking. We also like to add paper pulp. The exact mix depends on your soil and your application.
Consistencies of Cob (and the benefits of each)
History & Global Usage
Cob is pretty much the same as adobe. It has many names throughout the world. Mud and straw bricks were used to build Egyptian cities in biblical times. Great temples, great walls and humble homes have often been made of cob. Even today, over 1/2 of the world's population lives in earthen shelters.
How Cob Got It’s Name
Cob is an old English word meaning "lump" or "loaf". About 500 years ago in Devon county in England they discovered that their soil, when mixed with straw, was just right to build houses. The guy on top of the wall would tromp a lump of the mixture onto the wall and then holler down to the guys in the mixing pit; "Toss me another cob."
