After reading Scott Rao's book Everything but Espresso I was curious what the temperature looked like at various points in the coffee mound of my Chemex, so I wired it up with thermocouples to take a look.
What I found was interesting. First of all, and not so surprising, the Chemex bleeds heat pretty readily. Even though I preheat it and keep it on a towel rather than the stone counter-top after adding the last of the water, the entire apparatus still loses heat quickly. For this reason, you should decant your Chemex immediately after brewing. More on this in a minute.
Another thing I found is there is a difference in temperature stability at different levels or strata in the coffee mound; the lower regions of the mound seem to be more stable than the upper regions. This could be due to 1) an insulation effect of all the coffee above the lowest strata protecting from heat loss and 2) conversely, the upper strata absorbing some of the initial heat from a fresh pour.
In the graph above, T3 is a probe inserted into the brewed coffee and it looks like there's a point where it becomes very difficult to raise the temperature of the brewed coffee.
Getting back to the fact that the Chemex doesn't hold heat well; after I brewed the coffee for the test above, I continued to collect temperature data from the brewed coffee in the Chemex and also started collecting the temperature of the coffee in a coffee mug as I drank it slowly, as I normally would. What I found is that the Chemex loses heat just as fast as the coffee mug. For this reason, the post could have been titled "Your Chemex Is Not A Decanter." There are glass lids available for them and I wonder what affect a lid would have.
What does all this mean? Well we know to decant the Chemex to an insulated decanter immediately after brewing for sure. We also know that we should keep the fresh water as close to the top of the recommended temperature range as possible; around 210° F. That will help maintain higher average temps throughout the coffee mound. We also know that preheating everything — mugs and Chemex — helps maintain higher temps as well, though I didn't illustrate that here (next time maybe I'll do a test first without preheating and then one with preheating).
To wrap it all up nicely with bullet points;
- Brew with water just below the boiling point, or about 210°F
- If you can, keep your kettle on the fire between pours
- Decant the entire Chemex after it's brewed to maintain a good temperature longer
- Preheat your Chemex and your mugs