all glass brewing

Pure Over Brewing

The Pure Over brewing balanced on my V60 jug. It may seem an odd thing to do, but brewing into a clear glass container that can then be poured into a mug makes a better coffee. Firstly, the brew speed can more easily be monitored, and secondly, any fines that do fall through the glass filter basket are left in the bottom of the jug and do not make it into the final coffee cup.

The kickstarter project promised an all glass coffee drip-brewer without the need for paper filters: great coffee and an elegant brewer, all without waste. But how does the Pure Over perform in practise?

Created by coffee loving glass blower, Etai Rahmil in Portland, Oregon, prototypes of the Pure Over were developed with “The Crucible”, a non-profit art-school in the area. The Pure Over was designed partly to avoid the need for disposable coffee filters during coffee brewing. Although the 275bn disposable filters/year claimed in the kickstarter video sounds an overly high estimate of the number of filters used (though do let me know if you have a referenced value for this), it is true that disposable filters do come with an environmental cost, which could build up and be appreciable. So, if we can reduce our impact on that, it would be a good thing to do.

Now available to purchase online, I got my Pure Over in the initial Kickstarter campaign. When it arrived in early March 2021, everything about it was elegant: the glass had an aesthetic to it that was quite striking. It is easy to understand how the inventors of the Pure Over can describe their ambition as “to make our world a more meaningful and beautiful place”. But there was an immediate puzzle: the holes for the filtration basket seemed larger than I would have expected, would this really work?

I have in the past tried changing a Chemex paper filter for a metal Kone in an effort to reduce my use of paper filters. However, I never got on with the Kone. The filter in the metal was fine enough that the coffee grounds became stuck in it and it was consequently a bit of a pain to clean. At the same time, I never managed to optimise the cup it brewed. I should say that some people have found metal filters and the Kone great products, but I was not one of them. Seeing the glass filter basket therefore made me concerned that this elegant brewer would be pleasing to the eye but never to the palette.

I am happy to say that I was wrong. The Pure Over can make a great cup of coffee and look good too, but it does have a couple of quirks.

The funnel from the Aeropress is brilliant at directing the coffee grounds into the base of the Pour Over. Filling the grinds over a bowl means that any fines that fall through the glass holes can be rescued and put back into the top of the coffee bed. Thereafter the bed is quite stable.

Firstly the grind. The temptation (mentioned in some of the user-reviews of the Pure Over) is to assume that because the holes in the glass filter basket are quite large, a fairly coarse grind would be preferable so that the coffee does not fall through. This is a mistake. The Pure Over works because the water filters through the coffee bed. When the grind is too coarse, rather than produce a thick matrix of coffee for the water to percolate through, the grounds cannot pack closely and what happens is that a large amount of empty space opens up through the coffee percolation ‘bed’. This means that the water flows through the coffee bed too quickly, barely extracting any of the flavour compounds. The resultant cup is weak and unpleasant. If the grind is too fine on the other hand, it will indeed fall through the holes which ultimately block during brewing and the coffee becomes over extracted. The answer is to use a fairly fine grind but not too fine, I use ever so slightly coarser than the grind I use for making V60s. Of course, some coffee grinds do fall through initially, but if you hold the Pure Over over a bowl while you put the coffee grounds into it, you can then catch those that fall through and put them back in at the top. As these are finer grinds anyway, this has the effect of blocking some of the holes (vacancies) that form in the coffee bed and enhances the extraction of the coffee.

Secondly, the part-filter, part-immersion style of the Pure Over means that the water temperature is critical. Because you are using a fairly fine grind within what is partially an immersion brewer, using water that is too hot can result in the coffee being over extracted and bitter. Therefore, in addition to playing with the grind size, it is important to experiment with the brew temperature.

Lastly, the Pure Over comes with a diffusion basket which slows the pour of the water and spreads it over the coffee grounds. This turns out to be important because if you pour the water directly from a kettle it can lead to cratering within the coffee bed and result in a non-uniform percolation through the bed.

The diffuser on top of the Pour Over. The design is supposed to reduce the speed at which the water lands on the coffee bed as well as distributing the pour across the whole coffee bed. There is a lot of physics here, it will have to wait for another post.

When you have optimised these parameters (grind size, water temperature and speed of pour through the diffusion basket), the resultant cup is very much worth it. I found the coffees I made with it to have a character similar to the character that was apparent when I brewed with the V60 and different to the character that the coffee acquired when I brewed the same coffee with an Aeropress. The oils and some fines do come through, which is why I brew the Pure Over into my V60 jug and then pour that into my mug. This has the dual benefit of my being able to see how fast the coffee is filtering through the Pure Over basket and it resulting in a ‘cleaner’ cup as the fines are left at the bottom of the jug when I pour the coffee into the cup.

Over all, a really good cup of filter coffee without a filter. You can read another review of it in Barista Magazine here.

There is additionally a lot of physics involved in how the coffee brews. Although I didn’t mention it here, there is a link to traffic jams and filtration, a link to some novel methods now used in the organic farming of coffee beans and a connection to steam engines. There are also other links that I think do help to contribute to a more meaningful and beautiful world, so please do return in future weeks for an exploration of some of the physics involved in this interesting new addition to coffee brewing.