Plaster weight experiment

I performed this experiment to determine how the weight of cured and dried plaster compared to the weight of plaster powder I put in.  I'm using Hydrostone Gypsum Cement for my plaster.  If you're using a different kind of plaster your results may vary.

Materials

• 3 small plastic cups

• 3 spoons

• Hydrostone plaster

• A scale accurate to 0.01 grams

The Experiment

1. I weighed the empty cup and spoon to get the weight of all the materials other than the plaster

2. I weighed out 25 grams of Hydrostone.  If I didn't hit 25 grams exactly I got within 0.1 grams and called it good

3. I measured out 10 grams of water

4. Mix the plaster like I normally would and left the spoon in so there wouldn't be any loss of plaster.

5. I weighed the mixed plaster while it was still wet.

6. I put the cups under a fan for 36 hours to ensure they got completely cured and dried

7. I weighed the cups and subtracted the weight of the spoon

The Results

Cup 1:
8.88g weight of empty cup and spoon
25.06g dry plaster
10.03g water
44.21g Total weight wet
38.34g Total weight dry
29.46g Cured Plaster
Ratio: 1:1.176 dry : cured

Cup 2:
8.96g weight of empty cup and spoon
25.01g dry plaster
10.00g water
44.18g Total weight wet
38.38g Total weight dry
29.42g Cured Plaster
Ratio: 1:1.176 dry : cured

Cup 3:
8.98g weight of empty cup and spoon
25.05g dry plaster
10.05g water
44.12g Total weight wet
38.48g Total weight dry
29.5g Cured Plaster
Ratio: 1:1.178 dry : cured
 

The final results were consistent across all 3 cups.  The cured plaster was 17% heavier than the weight of the dry plaster.  I was surprised that it was so much higher but it was very consistent across all 3 cups.  The average ratio was 1.17666 to 1 cured plaster to dry plaster.  I'm going with 17% heavier because the .66% is negligible.

How to use this ratio:
Final piece weight: 900g
ratio: 1.17
cost of plaster: $0.66 / 1 kg

weight of final piece / 1.17 = weight of plaster used to make the piece.
900 / 1.17 = 769.23 (round to 769)
769g = 0.769kg
0.769 * 0.66 = $0.50754 which is the cost in plaster for the final piece.

Note:  This calculation for the cost of plaster used only works if the piece is a solid casting of plaster.  If you assemble the final piece from a lot of castings this will not account for the weight of glue.