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August 23, 2019 /
Steel, Glass, Ceramic?

What’s the most temperature-stable material to use when brewing?

The short answer is: plastic cones are best. They absorb heat from the brew water less quickly, absorb less heat overall, and lose that heat to the air more slowly. This conclusion may seem disturbing to some readers who are focussed on the growing problem around plastic pollution. We are pleased to announce that we have commenced work on a course called Cafe Sustainability that will look to properly unravel these issues. Furthermore, the growth of BH Unlimited means that our subscribers are helping make this important course open-access. That’s right, the Cafe Sustainability course will be free.

To really understand why plastic is the best choice of materials from a brewing perspective, we need to delve into a little physics. There are three things that affect how much heat you lose via the cone:

  • Conductivity – which measures how quickly the heat passes into and through the cone
  • Specific Heat Capacity – how much heat energy it takes to change the temperature of the cone
  • Heat loss at the surface – how quickly the cone gives up heat to the atmosphere

 

Conductivity

You probably remember from high school science lessons that plastic is a good insulator, and metal is a good conductor. Glass and ceramic fall somewhere in between. That tells us that plastic wins on conductivity, at least – but by how much?

Material Acrylic Glass Porcelain Stainless Steel
Conductivity (W/mK) 0.2 1 4-5 16

(Source: engineeringtoolbox.com)

As you can see, porcelain is 20–25 times as conductive as acrylic (a typical plastic). This means that the heat will travel out of your brew and into the cone much more quickly.

 

Specific Heat

Next, we consider how much energy the cone is capable of absorbing. This is called specific heat, and is measured in joules per kg per degree – in other words, how many joules of energy it takes to change the temperature of one kilogram of the material by one degree.

Material Acrylic Glass Porcelain Stainless Steel
Specific heat (J/kgC) 1250 753 1085 490

So plastic takes more heat energy per kilogram to increase in temperature by a given amount. However, a typical ceramic cone weighs four times as much as a plastic cone, so it will absorb around 3.5 times as much heat overall for the same temperature change.

This one often trips people up: it’s common to hear baristas say they prefer ceramic cones ‘because they hold more heat’. However, this is actually a bad thing, as it means it’s taking more heat out of the brew water.

 

Heat Loss at the Surface

Finally, heat leaves the cone into the atmosphere either by convection or radiation. The rate of convection depends on the surface temperature. More conductive materials will pass heat energy to the surface more quickly. Once the heat is at the surface, materials with lower specific heat will get hotter for a given amount of heat transfer. So plastic, with its lower conductivity and higher specific heat, will lose considerably less energy to convection than the other materials.

The rate of heat loss by radiation depends not just on the material but its structure, including how polished it is, as well as the surface temperature, so is hard to estimate by calculation. Glass, porcelain and plastic will all radiate a similar amount for a given temperature. Steel will radiate rather less, but this is offset by the fact that the high conductivity and low specific heat means that the surface temperature will get hotter much more quickly. Furthermore, the maximum heat loss by radiation is about half that of convection.

 

What About Insulated Cones?

Air is a better insulator than any of these materials, with a conductivity of around 0.02 W/mK. Some cone designs exist that take advantage of this, using either a double wall to trap a layer of air, or an open mesh-type structure to minimise the amount of material in contact with the filter, leaving it open to the air. These designs may help retain heat to some extent – however even in these cases the best material to make them from would still be plastic.

In the double-walled glass cones, the mass of glass will still absorb a lot of heat before the air layer comes into play. A plastic double-walled cone would work much better.

In the metal mesh-type cones, the surface area of the metal is still significant, which means it will still draw some heat away from the brew and dissipate it to the atmosphere. There will also be additional heat loss at the outer filter surface due to evaporation, which is a highly efficient way of transferring heat to the atmosphere. Ultimately, using a material closer to expanded polystyrene would do the same job much better and more cheaply.

 

Conclusion

So based on these three factors, plastic cones win every time — they absorb heat from the brew water less quickly, absorb less heat overall, and lose that heat to the air more slowly. The design of the cone, particularly its weight and surface area, play a part, but whatever design choices are made, plastic is the logical material to use.

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Steven Abbott
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Steven Abbott

The point about ceramics “retaining the heat” is valid if the cone has been pre-heated with hot water. However, it looks likely that the largest single environmental impact from coffee making is the heating of water, so pre-heating a ceramic cone may turn out to be more damaging than any problem of plastic. Similarly, the energy needed to make a simple plastic cone seems to me to be far less than a complex double-walled ceramic cone. And although I can imagine ways to extract value from a used plastic cone, maybe a used ceramic cone will have to go into… Read more »

Robert Gardner
Guest
Robert Gardner

The problem is that it is difficult to “pre heat” a ceramic cone because it loses that heat almost immediately. And then when you add brew water, heat is drawn from the water again to heat up the cone. I actually measured this myself several years ago, and documented the process in an article on reddit, which you can find by googling for “reddit v60 showdown”.

chris lehmann
Guest
chris lehmann

I use a plastic cone so this is not an big issue to me, but i put my porcellain v60 on the boiling kettle for one or two mitnutes and the steam was heating the v60 way better than just rinsing the paper.

Alice Mecklenburg
Guest
Alice Mecklenburg

This is a great scientific analysis of an issue that many of us have been grappling with. Although reducing plastic waste overall is a valid pursuit, we cannot deny that plastic cones simply work better than all other alternatives available to us.

Danielle
Guest
Danielle

While I prefer glass to brew, I only use stainless steel to hold and drink my coffee during work hours. Here is some information I found. I hope this helps https://bit.ly/2lZORny

Rose Trinh
Guest
Rose Trinh

Had this discussion about plastics a few times and why plastic is the better brew choice but at the same time, acrylics have a melting point of 160C which, no, we wouldn’t brew that high but despite the material being BPA-free, if we’re consistently brewing at hot temperatures (not saying 160C hot, but regular brew temps ranging from 85-95 for example) over and over again, it makes you wonder what the consequences could possibly be? That being said, there is also the impact that manufacturing acrylics produces highly toxic chemicals to those who work to make such a material and… Read more »

alexbitsios-esposito
Member
alexbitsios-esposito

Thanks for another great article. My preferences for brewer material are in this order: 1. Plastic 2. Stainless 3. Glass 4. and finally ceramic. You could argue from your results that glass may perform better than stainless, however, in commercial environments, glass and ceramic must be replaced and have a higher lifetime cost – and i would therefore expect a higher carbon footprint. My suggestion to people with glass or ceramic brewers at home is to pop the lid off the kettle whilst it is boiling and replace the lid with the brewing device to reduce the amount of energy… Read more »

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