Do you know how to filter stuff? Does anyone? Well, if you keep
reading this page, you are going to know a great deal about it.
Do you use a "coffee Maker"? If so, you already know
a little bit about filtering. Here in Central America the standard
way to make coffee is to simply put some coffee in a "bolsa",
a sort of fuzzy cloth bag and pour hot water through it. I think
it is the ultimate coffee maker. It costs about $0.20, and out-performs
any other technique that I know of. Just as an aside, do you know
what is the worst way to make coffee? A damned percolator. I think
my parents just rolled over and screamed at me.
There are so many facets to filtering; it is hard to know where to start.
What is filtering, anyway? Well, it’s a little science, a little knowledge,
and a little art. That’s right, art! To define the term we could say that
it any method that involves separating particles of different sizes by physically
screening them through some material. Are you happy with that? I’m not. Why?
Because when I said “material” you were thinking of pouring stuff through paper,
cloth, a wire screen, or a bed of sand, weren’t you? Yeah, you were!
You are right as far as it goes but there are other types of filters. Suppose
that you had a mixture of pumice and silica sand. How could you put this mixture
through a filter and separate them? We assume that the particles are of
the same size. What I would do is to simply dump the mixture into a bucket of water.
The water becomes a “filter medium”, not by size, but by density. The low-density
pumice floats and the dense silica sinks. We separated it with a density filter.
We could make an “electrical” filter. I could make a device that would separate
things by the “charge” they carry. Positively charged things end up in this pot
and negatively charged stuff ends up in that one. YEAH, YEAH, I know we don’t
usually call this type of thing “filtering”, O.K., so it’s all “separation
technology” but on this site we are going to consider it as “filtering”. O.K. by you?
We are not going to consider all these strange methods on this page. Maybe on some
other page we will get into chromatography, electrophoresis, density gradient separation, etc.
Hey guys, we are in MY field now. This is what I did!
This is probably the most used filtering method for small quantities
of slurry. It is also the method that is most used, most convenient,
and most practical for most of us prospectors. We know how to do
that, right? The first point I want to make is what kind of paper
are you going to use? Newspaper? Coffee filter paper? Paper towels?
Toilet paper? Or, Real scientific "designer" paper?
It doesn't matter which you use as long as it works for the
stuff you are trying to separate. This is where the "art"
comes in. You must pick the right kind of paper for the job you
are trying to do.
There are literally hundreds of types of filter paper sold by scientific
supply companies. They all have unique characteristics that make
one better than the other for your purpose. There are "hard"
papers, "slow" papers, "fine" papers, "open"
papers, etc, etc. Hang with me, we are going to try to sort some
of it out for you. When I had my labs, I probably had 150 different
kinds/sizes of filter paper on hand. It was necessary to get the
job done. Correct filtering is not a simple process and if you don't
do it correctly you are going to spend lots of time and energy that
could be better spent out in the river. A filter that is not right
for your purpose is one of the most frustrating things imaginable.
On this page I am going to talk about filter paper and I will, for
the most part, talk about "Whatman" paper. There are
other Manufacturers with products just as good, for example, Schleicher
& Schull. It's just that I am more familiar with Whatman.
Before we get into what paper is better for what I would like for
you to do a little experiment. It's not complicated, humor
me. I want you to find 3 funnels. Take 3 pieces of coffee filter
paper (or any other) and fold it in half twice. Open it so that
you have a single layer of paper on one side and three layers on
the other. Put these cones into the funnels and put the funnels
into some container to catch the liquid. Now I want you to take
about one tablespoonful of flour in a small pot or frying pan. Add
a little cold water and mix well, add more water, repeat until you
have the flour in a homogeneous suspension (no lumps). Add water
until you have about a quart of suspension. Now measure out one
cupful of this stuff and heat it until it just starts to boil and
let it cool to room temperature. In your first filter put water,
in the second put the non-heated flour suspension, and in the third
the heated flour suspension. Now I want you sit back and see how
the three filtrations behave. If all is as it should be, the water
will drip through pretty fast, the non-heated one will start fast
and slow to a slow drip. The heated one will start slow and slow
even more to a near dead stop.
The point of this is not just to demonstrate that some things filter
faster than others, you already knew that. I want to try to explain
why it happened that way so that you can begin to get some insight
into what might happen before you try filter something. Sure, you
are not going to be able to predict with 100% accuracy what will
happen, but with a little understanding, you can increase your chances
of picking the right type of filter for the job at hand.
Any filter that is separating things by particle size is composed
of a "screen". It can be paper, sintered glass, wire
mesh, or other. As long as the holes in the filter remain open,
not plugged up, the liquid will run through pretty fast. The larger
the holes, the faster the liquid will pass. If I took a frying pan
and drilled 3/8 inch holes in the bottom, dumped in a handful of
marbles and a gallon of water the water would immediately start
to gush through the holes. However, as soon as the marbles had a
chance to roll around a bit, they would find themselves a hole to
sit in and that hole would be sealed off and no longer available
as an exit for the water. As more marbles found a hole the water
flow would slow until the last hole had a marble in it and the water
could no longer pass through. Everything stops!
One other concept that I would like for you to grasp is similar
but not quite the same thing. If in that same frying pan, I dumped
in a lot of water that had a paper thin sheet of Jello floating
around in it, just about the same thing would happen. As soon as
the Jello got layered on the bottom, the water could no longer get
through. This is what happens to filters 90% of the time when they
are not working properly. You might be thinking, so what, nothing
you can do about it! There are lots of things that you can do about
it and we will get to them. After all, that's the whole point
of this webpage.
In the experiment above, as you would expect, the water having
no suspended particles to stop up the pores in the paper, flowed
right through. The suspended flour (starch granules) took a little
time but just like the marbles in the frying pan, eventually found
a pore and closed it off. The heated flour now has a high proportion
of soluble starch in it. It's the stuff your grandmother used
to starch Granpa' shirts. It's sticky, gelatinous stuff,
like Jello. This sort of stuff doesn't really have to find
a hole to plug up, it just sort of throws a blanket of sticky goo
over the filter paper and shuts it down.
ust a general information note at this point. If you want to buy
some general use filter paper to use around the basement or out
in the field I am going to make a couple of general recommendations.
Without giving all the characteristics of these papers let me say
that for general use you will find the following to be very useful.
- Whatman #1 This is a fairly fast paper that will stop particles
of about 11 microns (millionths of a meter)
- Whatman #4 This is a very fast paper that will retain particles
of 20-25 microns.
- Whatman #41 This is a fast paper that retains particles of 20-25
microns, It is particularly suited for filtration of gelatinous
- Whatman #42 This is a very slow paper. It will stop particles
of about 2.5 microns. Almost nothing gets through it.
- Whatman #541 This paper is very fast, stops particles of 20-25
microns. Its difference from #4 is that it can be used with very
strong solutions of acid and base.
Always buy filter paper in circles larger than you think you might
need. It’s easy to cut them down but awfully hard to stretch them.
Also, don’t buy “fluted” paper. You can flute it yourself simply
by folding it half as many times as you can and when you open it,
you have fluted paper. That makes filtering go quite a bit faster
due to the increased surface area exposed to the liquid.
Did you know that you can't fold a piece of paper in half
more than 7 times? No matter how big the paper or how thick or thin
it is. Try it!
a number of ways to filter things that just don’t want to be filtered.
Once a sticky material or any other that tends to stop up the paper
gets itself into the pores of the paper, everything stops. You can
try to suck it through or you can put as much pressure on it as
you like but it will only aggravate the situation. This treatment
simply drives the particles or other tighter into the pores. So,
what can a feller do? He can prevent those particles from ever getting
into the pores to start with. There are a number of ways to do this
but the simplest and most common is to employ materials called “Filter
What is a "filter aid?" Well, there are many materials
used for this purpose. You can often use "pulped paper".
That is, filter paper that has been ground or shredded. Sometimes
you could use something like clean silica sand. The most commonly
used filter aid is probably stuff called "diatomaceous earth".
So what is that, you say? It is the skeletons of microscopic marine
animals called Diatomes. As these animals die they settle to the
bottom and collect in layers where all the organic body parts decay
leaving the skeletons that contain billions of cavities, cracks,
pores, etc. This, of course, results in extremely high surface area
on irregularly shaped particles that cannot pack or compress.
There are two basic ways to use filter aids. The right way, that
is, of course, our way, and there is a method used by other less
knowledgeable folks. We will get to that but lets just say that
filter aids, when mixed with a liquid sample tends to provide surface
for particles, crystals, gooy stuff, whatever, to get hung up in.
It keeps the particles from getting together at the paper surface
and stopping up the pores. Sort of keeps all the nasty stuff at
arms-length from each other.
suppose that I should explain here that these aids are usually
used in vacuum filters. That is a filter that has an evacuated flask
attached to the stem of the funnel so that the liquid is being sucked
down through the filter paper. This not completely necessary. You
can do it by gravity but will probably find that you must use a
little more filter aid than you would with vacuum and it will be
BTW -- Does anyone know of any other word in the English language
that contains two U's in sequence??
O.K., how to do it. I have seen Ph.D.s in the lab use filter
aid by slurrying it with some water, pour it into the filter, suck
out excess water, and then pour the sample to be filtered on top
of it. He has just successfully defeated the whole purpose of using
the filter aid. That ain't the way to do it. The way we Basement
Chemists are going to do it is to put the filter aid directly into
the liquid sample, slurry it around and then pour it into the funnel.
How much do we use for any given volume? I don't know! You
have to experiment. It depends on what sort of nasty stuff is in
your sample. Just guess. If you have 300 ml try about a teaspoonful
of aid. You will know if it works or not. If the goo still accumulates
on top of the filter aid you will have to use a bit more. Just recover
all of the filter aid and re-slurry it. Add another teaspoonful
of filter aid and try it again. I told you that there was a little
art involved. One word of caution, once you pour the slurry into
the filter, DON'T STIR IT. Don't even scratch around
on the surface of the packed filter aid. It almost never helps and
it can cause things to slow even more.
I must comment on a human phenomenon that in the case of filtering
is self-defeating. Here in Central America we have many Diesel vehicles.
The fuel here is probably not quite as clean as that you are accustomed
to in the USA. It is the custom here to put extra in-line fuel filters
on all cars etc. Gentlemen, this is dumb. If you put two identical
filters in line you do NOT get twice as good filtering. If a particle
passes the first filter, it can pass the second. I once bought a
used Land Cruiser here upon which I eventually found five in-line
filters. Actually what they do here is when you tell the guy to
change your filter he doesn't remove the old one. He simply
puts another one before it. The same suspect filter is still in
the line and still capable of slowing fuel flow. Never will understand
that line of thinking. Same thing with paper filters. The only reason
to double-up is to provide physical strength. You will never need
it with professional paper.
If you anticipate doing very much filtering I highly recommend
that you consider the advantages of using a vacuum filter. The most
common, and, most useful of these is the Buchner Funnel. It is usually
made of porcelain but you can get them in plastic too. It is simply
a funnel with a flat plate or bed on which you can lay a piece of
filter paper and suck the liquid through the paper with a vacuum.
You will find this devise will make life much more pleasant if you
are doing much filtering. Actually, there are two pieces to this
apparatus. You must have the Buchner funnel and you must also have
a Filter Flask. The latter is the catch vessel and the vessel that
is attached to your vacuum source. The other thing that you will
need is a rubber stopper with a hole in it to seal the joint between
the funnel and the flask. Alternatively, you can buy a set of “adapters”,
just little rings of rubber that will serve very nicely. None of
this stuff is very expensive. The following drawing will, I hope,
illustrate the device that we are talking about.
Now if you decide you want to invest in one of these things, you
must decide what size do you want. Well, for average use, I would
buy a filter funnel of about 15cm diameter and a flask of 1liter
capacity. That is up to you and your needs.
O.K., If you buy a vacuum filter you will still have to have a
way to produce a vacuum. There is a device that I highly recommend
that is called a Water Entrainment Pump. It is a simple-minded device
that you can attach to any water faucet to produce a suprisingly
high vacuum. They are of metal or plastic and they are really useful.
They are also inexpensive. This is the vacuum source that the Buchner
system was designed to work with.
Now, for those of you who would like a vacuum source for less or
maybe free, there is another option. Go down to some place that
has used or worn out refrigerators. Some that they about to junk.
Find one that still runs but is nevertheless ready to be scrapped.
Get the compressor from it. Hook it up and you will find that frequently
it will still produce a credible vacuum. You can use it!
|Sintered Glass Filters
I think that I must mention that there are filters that are made
exactly like the Buchner Funnel but instead of having discrete holes
for the liquid to flow through, have a filter bed of porous glass.
If you should end up with one of these you will find that after
a time the filter will no longer allow even water to pass through
it. It will laugh at all your attempts to clean it. However, we
Basement Chemists are not to be intimidated by such trifles, are
When this happens we must resort to our above average knowledge
of redox reactions. The first thing to try is to simply dissolve
some Nitrate in water and try your best to suck some of it into
the sintered glass. Usually you can. Now take the filter (assuming
that it is NOT plastic) and put it in the oven. Get it as hot as
possible for an hour. This will probably remove all the carbonaceous
material that usually causes the problem. You might even have to
If that don't work we go to a more extreme method to save
our filter. For this you will need a little Sodium Dichromate and
some concentrated Sulfuric Acid. To the acid add, with stirring,
the Dichromate crystals until you get a nice dark red solution.
DON'T GET THIS ON YOU!! It will clean your bones too. Just
soak the offending filter for a few hours or days, wash it and see
if that got it.
There is another general cleaning solution that you can use to
clean this sort of thing. As in the above, I can't remember
the exact proportions but just make up a solution of Lye (caustic).
About 30 grams/liter, add strong Potassium Permanganate solution,
with stirring, until you get a dark blue solution. Like the Dichromate,
DON'T GET IT ON YOU!! It will do a job on you too. As above,
just dunk the filter in it and forget it for awhile.
Both of these solutions are used extensively to clean glassware
in laboratories. They can be used on pipettes, general glassware,
ceramic, porcelain, sintered glass, gas spargers, etc.
Ain’t All This Fascinating??