Fig 1. Bentonite, near Cathedral Valley, Utah, USA
(scale: the 'kernels' are about 2 cm in diameter)

Bentonite is a type of clay, weathered from volcanic ash. It has a remarkable ability to absorb water, swelling to many times its dry volume. Plants have a hard time growing in bentonite because the extreme swelling and shrinking tears apart their roots.

Production notes

Fig 2. Camera and lighting

This time lapse video was produced from a sequence of still images taken at 3 minute intervals by a Canon A75 attached to a computer, using Canon's "Remote Capture" software (part of 'ZoomBrowser'). The images were merged into a video using QuickTime Pro, and exported using H.264 compression.

The bentonite was prepared by grinding a few 'kernels' into powder using a mortar and pestle. The powder was sprinkled slowly into a tea cup containing about 0.5cm of water, trying to distribute the powder as evenly as possible. When the bentonite powder grains strike the water, they swell and sink, gradually building up until the water is entirely absorbed. Powder was applied until there was a very faint, thin layer of dry powder. Then the surface was misted with water from a sprayer, to moisten these few surface grains. This preparation method seemed to result in lower density bentonite (ie., more water per bentonite) than simply stirring bentonite into water.

Incandescent lighting was used (a 15W bulb, to reduce heat); CFL bulbs can create flicker. Custom white balance was set using a white sheet of paper. Macro mode, medium zoom. The photography was done in a dark basement so as to be unaffected by night/day light cycles.

The camera was powered by an AC adapter.

The slight movement of the tea cup over the ~15 hour filming period might be due to fluctuations of the basement temperature affecting the tripod and/or support for the tea cup. The filming was done in January in Ottawa and the outdoor temperature was low, causing the house heating system to run with fairly high duty cycle. The relative humidity was about 20%.

To generate the small 'clock', I wrote a Photoshop script (addClock.jsx) that uses time stamps of the image files; an appropriate clock image is added to each file prior to processing by QuickTime. Other clock images and hands could be used by simply modifying the base clock image in clock.psd. The Photoshop script language documentation is in a sub-folder where Abode s/w is installed; the script development environment (editor and debugger), called Extended Script Toolkit 2 (part of PS CS3), is very helpful.

Jan 2008