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[Cob] just cob until it feels rightShannon C. Dealy dealy at deatech.comMon Feb 28 12:07:47 CST 2005
On Mon, 28 Feb 2005, karl and tabitha o'melay wrote: > while attending a cob party (me learning to cob) a question arose. if > the sand is course do you need to add more or less clay proportionally > than fine sand? > > borrowing a frequent analogy > if > the clay is mortar > the sand is brick > the straw is rebar > > then, logically > if the brick is really large, less mortar would be used to assemble a > given wall? > > or does that not hold true in this case? [snip] No. The sand might be "brick", but it's a bunch of broken pieces of brick so depending on "average" shape and distribution of sizes, it could require either more or less clay. To give an extremely oversimplified example, imagine a set of wooden building blocks like the ones children play with, and how much clay it would take to mortar them together into a cube one foot on each side. Then imagine how much it would take to mortar together a bunch of oranges to make the same cube. Now imagine again using a mix of oranges and marbles, less mortar, but still the building blocks win. It is the shapes and size distribution together that make the difference, this is part of the reason that round beach sand should be avoided if possible (the other being that irregular shapes interlock better), as well as why it is recommended that if possible you use sand with a good range of grain sizes, the sand grains are stronger than the clay, so as your clay content increases beyond what is needed to bind the cob, the overall strength of the mix will tend to decrease. Generally this is not going to be critical with a reasonable mix, but you will definitely notice the difference if you start playing with really high clay mixes for a while. Shannon C. Dealy | DeaTech Research Inc. dealy at deatech.com | - Custom Software Development - | Embedded Systems, Real-time, Device Drivers Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications or: (541) 929-4089 | www.deatech.com
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