October 1996 Volume 19, Issue 10

Bottle Glass Absorbency

reprinted by permission from Ian Craig & Spencer Thomas, http://realbeer.com/ spencer/bottle.html

DATA

The data for this graph was gathered by Ian Craig <craigim@ garnet.berkeley.edu>. It consists of optical density readings every 2 nanometers from 190 nm (in the ultraviolet) to 802 nm (in the infrared). Density is the negative logarithm of the transmittance, so a density of 0 means 100% transmittance (-log(1.0) = 0), a density of 1 means 10% (-log(0.1) = 1), etc. The important bit is this: The higher the density, the less light gets through. The graph appears to top out around an OD of 4, corresponding to less than 1/10,000 of the original light getting through.

I have colored the portion of the axis corresponding to the visible portion of the spectrum. The critical part for beer skunking seems to be wavelengths shorter than about 500 nm, indicated by the vertical line crossing the whole graph. Note that the brown bottles (Red Nectar Ale and Samuel Adams) transmit a lot less light in this part of the spectrum than do the green (Rolling Rock) and clear (Samuel Smith's) bottles. The Elephant bottle, which is dark green, is in between.

Measurement process

Here's a description of the measurement process from Ian Craig: As for the procedure, I cut a hunk out of each bottle using a glass saw. The lab I work in has a UV/Vis absorption machine which I used to take the spectra.. It works in the following way: Light from a lamp (I think it's tungsten) produces a continuum of light with good intensity in the 190-820 nm range. The light is collimated and is split through a 50/50 beam splitter. One beam is sent through the sample and the other is sent around it. Both beams then hit a grating and are dispersed onto a dual diode array which measures the intensity of light at each wavelength. The data from the sample is then divided by the data from the reference to correct for variations in intensity of the lamp at different wavelengths. This array is then read by the computer and is output as a single column of numbers. I imported the five arrays into Excel and saved it as column delimited ASCII text which is the form you received it in.

The descriptors of each bottle is the brand of beer, although all beers may not be available in all places. The Elephant is dark green and the Red Nectar is an amber to light brown. The other are nationally available brands and so should be familiar.