Posted by: Brian | February 4, 2011

Tempering Pots – A Performance Experiment

Making Standard Bowls - Rachel and Alysia making all the bowls to a standard size and thickness. Rachel is using a round rock as a mold to insure all bowls are basically the same.

My Experimental Archaeology class ended the semester by designing and implementing their own experiments. I was very impressed with their results, especially given the limited time and facilities they had available. One group decided to explore the issue of temper in clay pots. They made four sets of bowls – one set tempered with sand, one with fiber (deer hair), one with shell, and one with grit. They fired their pots at Como Park, then brought them back to school to perform their experiments. They looked at differences in water absorption, thermal transference, and durability.

Bailey and Will organization the water absorption test. Theirs was a very simple experiment – they just filled up the bowls with an equal amount of water and timed how long until the water either evaporated or was absorbed into vessel walls. They did two trials – one with dry bowls, and one after the bowls had absorbed the first sample of water. It took between 13 and 45 minutes to complete the first trial, but over 5 hours to complete the second trial. I think the second trial was really important because it demonstrates that evaporation wasn’t a significant factor.

Bailey adding fiber temper to her clay. We used deer hair because I've seen Norton tradition pottery from Alaska tempered with caribou(?) hair. Bailey said it was hard to work the hair into the clay.

So what were their results?

The fiber tempered bowl was consistently the fastest absorber. The grit-tempered bowl was the slowest. The differences were fairly impressive. There was three hours difference between fiber and grit temper during the second trial. The sand and shell tempered bowls fell in the middle between the fiber and grit – with no substantial advantage between shell and sand.

Watching Water Evaporate: Bailey and Will undertaking the evaporation experiment. I believe watching water evaporate requires nerf guns for entertainment.

The second experiment evaluated the cooking performance capabilities of the different vessels. Alysia and Rachel used the stove top in their dorm kitchen (good thing there were no accidents) and timed how long it took to either bring water to a boil – or evaporate the water. The water in the sand tempered bowl was the quickest to heat up – with the water boiling after only 5 minutes and 35 seconds. The water in the other vessels never reached a rolling boil, but instead evaporated. The water lasted 6 minutes 30 seconds in the fiber tempered bowl, 10 minutes in the grit-tempered bowl, and 12 minutes in the shell tempered bowl.

For the final stage of their experiments on temper and vessel performance, the students decided to test for differences in durability. Nate used one set of bowls for this experiment and dropped them from different heights until they broke. The fiber tempered vessel was the most durable – although the differences were slight.

According to my students, if you wanted a clay vessel to store water, you’d temper it with grit (and definitely not hair). If you wanted a vessel that was efficient at bringing water to a boil when heated, you’d temper it with sand. And if you wanted a vessel that could best survive being dropped, you’d temper it with fiber. Intriguing results – especially for experiments that were conceived, designed, and implemented in just a few days.

I plan to write about some of the other experiments in my next post. Check out my Flickr site if you want to see more photographs.

Stove Top Experiments: Alysia and Rachel testing how well the vessels heat water. They reported that the hair tempered bowl smelled badly when heated.


Nate's Experiment - testing the durability of the bowls when dropped. We should have done this outdoors where the ground was a little more forgiving. Every vessel broke between 3 and 12 inches.

Nate's Results



  1. Wow – what a great course! Congratulations to you and your students – impressive, indeed.

  2. Pretty slick. Can you elaborate on what exactly “grit” is in this case? I always assumed it was synonymous with sand temper.

  3. Thanks Tim. Your blog and experiments have been an inspiration. I love your harpoon and ballistics gel. ( ).

    Adam – there does seem to be some differences in the use of the term ‘grit’. The Getty Museum’s Art and Architecture Thesaurus and other sources, however, define grit temper as crushed rock. See also the UM’s “Prehistoric Pottery of Minnesota” glossary

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