In A perfect filter spell. What knowledge could it bring? I described a magical filter. The common suggestion of filtering out metals (specifically gold) dissolved in water has been wracking my brain. How would I actually build an installation that gets me nuggets of gold out of the ocean water?

To build the installation I have created an in- and outlet for the water together with a bunch of filters. All of the filters are framed and rectangular. Half of them will block the passage of everything but all forms of pure gold. The other half is the opposite, blocking only the pure forms of gold but not anything else. The direction of travel is irrelevant to the filter as it works the same in either way. Any piping can be made with corrosive resistant steel. I have no access to engines so the whole installation needs to be gravity fed. (Do not worry about the outlet. That has been built already.)

As a reference to the power of the filter. It is similar to a Maxwell daemon. the difference is that my filter always works the same in both directions.

Some things that wouldn't work: Passing the water over a filter that lets gold through. And placing a filter blocking the flow that lets everything through but gold. The gold won't come out of solution in the first case and will form a solid plug of AuCl4 salts (probably Sodium tetrachloroaurate) blocking the flow in the second filter.

  • $\begingroup$ Does it need to be fully automated and tide-driven? Surely if you're harvesting gold from the ocean you can afford to send someone to go clean out the filter every so often. $\endgroup$
    – Cadence
    Commented Oct 25, 2021 at 17:00
  • 1
    $\begingroup$ There is indeed some gold dissolved in ocean water. The problem is that gold is for all practical purposes insolube in water, be it fresh or salt water. The concentration of gold in ocean water is about 10 picograms per litre, or about 1 gram of gold per 100 million tonnes. Even with a perfect filter you need to filter 3 billion tonnes of sea water to get one ounce of gold. Assuming that the filtering installation is fed by a conduit 100 meters wide and 10 meters tall through which water moves at 20 km/h, the installation will produce one measely ounce of gold every five days... $\endgroup$
    – AlexP
    Commented Oct 25, 2021 at 17:23
  • $\begingroup$ @AlexP i have plenty of time in my story. and the facility has very few spacial limitations. the problem is how to actually get the gold to separate from the water and collect in a bucket of sorts. $\endgroup$ Commented Oct 25, 2021 at 17:31
  • $\begingroup$ @Cadence it does not have to be fully automatic. but as AlexP points out, gold isn't all that common. there would have to be a compromise making the job doable and have a high throughput of water. $\endgroup$ Commented Oct 25, 2021 at 18:22
  • $\begingroup$ I invite you to read "Advice concerning questions asking HOW to implement a technological procedure or device" and its answers very carefully, then set your expectations for what kind of answers you'll get. $\endgroup$
    – JBH
    Commented Oct 26, 2021 at 0:10

3 Answers 3


You only need a gold blocking filter

You do not need a filter that blocks everything but gold at all. Generally speaking, filters have a tendency to cause adhesion with whatever they collect; so, backwards diffusion is not normally a problem. But, if your magic filter only blocks gold, and does not catch it, and you don't have a reliable flow of water then there is another very simple mechanical solution. If you place a 1-directional valve in a gravity trap, then whenever water is flowing through your system in the correct direction, the valve will open, the flow of water with press your gold against the filter preventing it from diffusing backwards. Then if the flow of water stops, or worse, goes in the wrong direction, then the valve will spring shut trapping the gold inside. Unlike a magic filter, a valve lets everything through (when open) so you don't get massive buildups of salt.

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That said, I would recommend a river over the ocean by a long shot.

  • It maintains a way better flow allowing a given size filter to do a lot more.
  • You can build a damn to make sure you always have an elevation gradient to work with so that it continues to work at all times, eliminating the need for a valve at all.
  • Rivers have orders of magnitude more gold in it than the ocean. Rivers have average gold concentrations of 1 to 5 ppt (parts per trillion) with up to 2,800 ppt in places with favorable mineral deposits. The Ocean has closer to 0.01 parts per trillion.
  • $\begingroup$ Just block the gold. Have a constant flow of water through the filter to prevent the accumulated gold from drifting away (it will be, literally, atom-sized fragments). Once your filter stops passing much water, it is full and can be harvested. You may want to put a second, reversed filter against the first to trap the gold dust during extraction. $\endgroup$
    – PcMan
    Commented Oct 26, 2021 at 6:49
  • $\begingroup$ @PcMan Thanks. I've improved my answer though to address backflow in a way that still only needs 1 magic filter. $\endgroup$
    – Nosajimiki
    Commented Oct 26, 2021 at 16:23

Shutdown the spell

When a specific weigth is reached by the filter that captures gold, it's removed and encased in a container. Inside this container you shutdown the spell leaving only the gold molecules trapped inside.


If these filters are magically perfect in whitelisting or blacklisting specific elements or compounds, then you could just have the inlet lead to a triangle with two magic filters, one which blacklists gold (lets everything but gold through) and one which whitelists gold (lets only gold and nothing else through. They are 90 degrees apart in an upside down triangle. While the gold filled water spills into the triangle, the water filters through the first filter and gold falls along out of the second filter.

If that doesn’t work and if the gold sticks to the filters, then you could simply stick blacklist gold filters through a stream of water. As gold sticks to the filter, people can pull the filter out and scrape off the raw gold before putting it back in the stream

  • $\begingroup$ That triangle would work for plenty of other materials but not for gold. Gold is only soluble when it has formed a complex with Chlorine. So the Chlorine would prevent the gold from going through the second filter. $\endgroup$ Commented Oct 25, 2021 at 17:26
  • $\begingroup$ how will you prevent the gold from diffusing backwards against the stream? $\endgroup$ Commented Oct 25, 2021 at 17:54
  • $\begingroup$ @PostlimFort Gravity. At least if I'm reading the answer correctly, gold falls from the filter into a container (and water falls from the other filter into some kind of return duct). $\endgroup$
    – Cadence
    Commented Oct 25, 2021 at 19:22
  • $\begingroup$ @PostlimFort Generally speaking filters have some level of adhesion with the elements that they filter. As long as gold sticks to the filter, then there should not be a concern. $\endgroup$
    – Nosajimiki
    Commented Oct 25, 2021 at 20:52
  • $\begingroup$ If you are worried about the Chloride compounds, then you can just add Chloride to the filter as well so it doesn’t go through the water filter and comes along with the gold filter and you can send the gold to a refinery plant or whatever you want to do with it $\endgroup$ Commented Oct 26, 2021 at 12:17

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