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It’s hard to remember where I’ve got to in all these endless series. About time for another instalment.

I’m sure you’re all just dying to learn more about kieselguhr filters and their use in the 1950’s. No? What is wrong with you people?

Let’s start with the construction of a kieselguhr filter:

“The kieselguhr filter is of a different construction. In one form it consists of a series of metal discs with central holes, stacked together to form a sort of candle. Small projections upon the sides of the discs result in the formation of very narrow channels between the outer edges of the discs and the edges at the hole. A number of these candles are housed in a totally enclosed cylinder. A definite amount of kieselguhr is mixed to a slurry with beer automatically in a mixing chamber and the mixture is forced edgewise through the discs from their periphery to the centre hole. The kieselguhr builds up in the small interstices between the discs, thus forming an efficient filter. The beer is circulated through the discs until it issues bright and then passes forward in the usual way. Different grades of kieselguhr are available to give either rough or polishing filtration as required.”
"Brewing Theory and Practice" by E. J. Jeffery, 1956, page 343.
That slurry of kieselguhr and beer sounds just lovely.

Searching the internet for kieselguhr, I discovered something even less appetising: that it may be adding arsenic to beer. A researcher at Weihenstephan examined beer for heavy metals and discovered it almost always contained arsenic. He reckons that the source is the kieselguhr used for filtration.

Seems like a good reason to stick to unfiltered beer.

Here’s what happened when the filter was filled with sludge:

“When the build up of deposit from the beer has clogged the filter, the beer is cut off and the filter mass can be readily removed by back-washing with hot water. A fresh filter mass can soon be deposited and filtration commenced again with very little interruption or the process. Different commercial plants have various ways of building up the filter mass, but the principle is the same. Usually the used kieselguhr is thrown away, but suggestions have been made for washing it in hot water or dilute caustic soda to dissolve the beer deposit so that it can be re-used. The most effective use of the earth filter is probably as a roughing filter followed by a plate filter as polisher. Thereby the advantages of the earth filter in dealing with a considerable amount of coarse deposit are combined with the probably greater effectiveness of the plate filter for sterile filtration. A very recent development in filter mediums is the production of a porous sheet made by a special process from polyvinyl chloride (P.V.C.). The sheet is very thin (like paper) but extremely tough, and filtration occurs mainly on the surface and not in the interior channels as in the more usual filter sheets. Consequently the deposit can be washed off and the life of the sheet is claimed to be very long. By the use of special gaskets it can be used in some filter presses originally designed for normal thick sheet filters. There has been little opportunity yet to try it out extensively, but preliminary trials have been very encouraging.”
"Brewing Theory and Practice" by E. J. Jeffery, 1956, pages 343 - 344.
Sounds very simple. Wash out the sludge with hot water and add another batch of the beer and kieselguhr slurry. Is it worthwhile washing kieselguhr? If there isn’t a huge amount used every time, unless it was very expensive stuff, I can’t see that it would be worth the trouble of washing it.

Using two different types of filters for rough filtering a polishing seems to make sense in practical terms. I wonder if people still do that? A quick look in Briggs reveals that the use of two filters is not recommended:

"Views vary considerably on the relative advantages of different types of filter. It is difficult to draw firm conclusions but some principles emerge. To avoid losses and to avoid oxygen pick-up, single pass filtration should be used wherever possible. On this basis powder filtration is usually regarded as being superior to sheet filtration."
"Brewing Science and Practice" by Dennis E. Briggs, Chris A. Boulton, Peter A. Brookes and Roger Stevens, 2000, page 581.
Though both plate filters and kieselguhr filters are still in use:

"Different mechanisms of filtration can be used:

· Sieving or surface filtration in which the particles are trapped in pores in the filter medium and retained in a layer. Filtration quality improves with time but the volume flow decreases continuously.

· Depth filtration in which a separation medium, e.g., kieselguhr is used on a support and which causes the particles in the beer to take a very elongated route through a large surface area. The particles are retained by mechanical sieving because of size and will gradually block the pores in the medium and so reduce flow rate and the particles can also be retained by adsorption as a result of electrical charge effects."
"Brewing Science and Practice" by Dennis E. Briggs, Chris A. Boulton, Peter A. Brookes and Roger Stevens, 2000, page 575.
Kieselguhr sounds like pretty nasty stuff. Not sure I’d want it in my brewery:

"However this substance is classified as highly dangerous when inhaled and can give rise to the disease of silicosis. Equipment is needed for automatic slitting of bags and transfer to slurry tanks to avoid manual handling. Uncalcined kieselguhr, prepared by drying at 400ºC (750ºF) represents only a moderate risk and is now usually preferred. However, some uncalcined kieselguhr can contain traces of iron and other metals."
"Brewing Science and Practice" by Dennis E. Briggs, Chris A. Boulton, Peter A. Brookes and Roger Stevens, 2000, page 577.
Finally a mention of centrifuging:

“Centrifuging is also used, mainly on the Continent, instead of filtration, but does not seem to have found much favour in this country.”
"Brewing Theory and Practice" by E. J. Jeffery, 1956, page 344.
Pretty sure that’s no longer true and that centrifuges are now common in the UK.

Still not quite finished with filtration.