Plaster

When constructing an adobe house you have to plaster the outside in order to make it weather proof. With this knowledge we knew that a plaster would be necessary in order to hold the water inside our adobe box. But which one?  Researching natural plasters we found that we could make some using clay, sand, straw and paper pulp (http://www.pequals.com/at/earthenplaster).  The materials that we had available to us site were: dirt rich in clay, very coarse sand, straw (finely ground with our hands), and paper pulp which we created from our old homework and the help of a blender.  We tested 8 types of plaster with different combinations of these materials to determine the recipe that would give us a smooth plaster with little or no cracking.

The table below lists the ingredients and results of the different plasters.  A thin layer of each test plaster was applied on the smooth side of an adobe brick, rubbed down with a wet hand, and left horizontally on the ground in direct sun.  Just for experimental purposes, a thick layer was also applied to a ceramic tile and also left in direct sun.  As we were in the middle of making different plasters we realized that tests 1-4 were already cracking.  Cracking plaster indicates too much clay where as a plaster that will not stick together indicates too much sand.  We started experimenting with extra sand and adding paper pulp.   The last 3 tests with extra sand were also applied on a standing adobe wall to test the difference between the plaster laying horizontally and vertically.

Our results varied due to a random rain storm which occurred only two hours after the tests were laid in the sun to dry.  24 hours after the tests began we were able to make conclusions about the different plasters.  All tests on the tiles remained to damp to see results.  Tests 1-5 produced major cracking.  For tests 6-8, all the test results on the adobe wall showed small fragmented cracks while the tests on the ground produced more cracks.  This could have been due to a thicker layer of plaster on the wall tests since thin layers of plaster tend to crack less.  We found that equal parts of sand and clay produced the best plaster with the least amount of cracks (tests 7-8).  The only difference between test 7 and 8 was the addition of paper pulp, which seemed to produce a smother finish.  Since test 7 had the least cracks and smoothest finish this was our chosen plaster:  3 parts clay, 3 parts sand, 1 part straw, 1 part paper pulp.

Test #

Ingredients

Observations/Notes

Results

1

4 parts clay

Very wet mixture, too much water.

Lots of cracking.

 

3 parts sand

 

 

 

1 part straw

 

 

2

4 clay

Less water, too much clay.

Lots of cracking.

 

2 sand

 

Cracking indicates too much clay.

 

1 straw

 

 

3

3 clay

Feels like more straw

Cracking.

 

2 sand

content per ratio.

 

 

1 straw

 

 

4

4 clay

Lots of straw, too much clay.

Cracking.

 

2 sand

Not a very smooth finish.

We need a higher sand ratio

 

2 straw

 

to prevent cracking.

5

5 clay

Pulp was way to wet, water

 

 

3 sand

needs to be squeezed out.

Cracking

 

2 straw

 

 

 

2 paper pulp

 

 

6

4 clay

Pulp was compressed together

Less cracking.

 

3 sand

to remove water.

 

 

1 straw

Looks very smooth

 

 

2 paper pulp

in the drying process.

 

7

3 clay

 

Very smooth, little to no cracking.

 

3 sand

Feels very sandy.

 

 

1 straw

 

 

 

2 paper pulp

 

 

8

3 clay

Very coarse

Little cracking.

 

3 sand

 

 

 

1 straw

 

 

All of plaster tests 

The three best tests (#6-8)

final paster inside

 

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