Separating mixtures
About three thousand years ago a group of people set up a camp on a hillside. They made huts with stone walls and turf roofs. They made clay pots to hold food and made axes and knives of iron. In those days, when things got broken, they were not taken to a tip. They were thrown away close by the home. As time passed people left the camp and it became derelict. The wood beams holding up the roof rotted away. The turf collapsed on the stone walls and soil gradually buried everything.

Archeologists are people who study how people lived in the past. Some archeologists discovered the ancient campsite and began to dig it out. They dug carefully so as not to damage anything buried in the soil. The stone walls of the huts came into view but the archeologists could not see the smaller items that the people used. The pottery would be broken but as it does not rot it could still be present somewhere. Iron knives and axes rust but some small pieces could still survive around the huts. To test these ideas, the archeologists made a sieve. It was rectangular, with high wooden sides to hold the soil, and about the size of a desktop. The sieve was supported by poles like the swings in a playground and could actually be swung to and fro.

The archeologists shovelled soil from the camp into the sieve and swung it between the poles. As the sieve moved to and fro, small lumps of soil fell through the holes. In a short time all the small lumps of soil had passed through the sieve. Left behind in the sieve were larger items such as gravel and pebbles. When the archeologists searched carefully through the items in the sieve they found pieces of pottery and small, very rusty pieces of iron which might have been from axes or knives. The sieve had saved the archeologists a great deal of time. If they had not used it they would have had to dig very carefully through the soil for weeks.

By sieving large amounts of soil many pieces of pottery were found. Some could be stuck together, like the pieces in a jigsaw, to show how the pottery looked before it was broken three thousand years ago.

How does a sieve work?
It has holes of a certain size. Objects smaller than the holes pass through. Objects larger than the holes stay in the sieve. The sieve is a device for separating objects of different sizes.

Where are sieves used?
They are used in the kitchen to sift powders such as flour. The flour is shaken in the sieve and the smallest grains pass through. Lumps, made by grains of flour sticking together, are left in the sieve and thrown away. If they were used with the rest of the flour they would spoil the baking. Sieves are also used in flourmills to separate the parts of the flour. Flour is made from grains of wheat that have been ground up. Besides the white powder there are larger particles called wheat germ and bran. The sieves are used to separate these larger parts to make white flour. Wholemeal flour contains all the parts of the wheat grain.

Are sieves used outside?
Yes. In addition to use on archeological sites they may be used to improve the soil in a garden. If the soil has a large number of stones and pebbles in it, a sieve can be used to remove them. A sieve can also be used to separate potatoes from the soil. On the beach some people use a sieve to collect cockles from the sand. The cockles are shellfish which some people like to eat.

Can sieves be different sizes?
Yes, they can. Scientists who study soil use a number of sieves. The sieves stack together like a pile of plates. The top sieve has a lid and the bottom sieve has a bowl fastened to it. Soil for sieving is put in the top sieve. The lid is put on and the whole pile of sieves is shaken for a few minutes. When the sieves are separated, each one contains particles of soil of a particular size. The largest particles are in the top sieve and the smallest particles are in the bowl below the bottom sieve. By looking at the amounts of the different particles in the soil, the soil scientist can predict how it will behave when it is wet and how well it will hold mineral food for plants.

Why is a filter paper like a sieve?
It is like a sieve because it has got holes in it. A sieve is made with wires that cross each other at right angles and the holes can be clearly seen. A filter paper is made from tiny fibres of wood which cross each other in all directions. These holes are tiny and can best be seen with a microscope. Both the sieve and the filter paper have holes but the holes in the filter paper are much smaller than the holes in the sieve.

Is a filter paper used in exactly the same way as a sieve?
No. A sieve is used to separate solid particles of different sizes. A filter paper is use to separate solid particles from a liquid. For example a filter paper is used to separate coffee grains from hot water.

Some people wear masks in dusty places? Is there a filter in the mask?
Yes, there is. It may be made of fine cloth. The holes in the filter are large enough to let air pass through them, so a person can breathe. They are small enough to trap particles of dust in the air, which may harm the throat and lungs.

Are filters always made of paper or cloth?
No. The water you draw from a tap has been filtered by sand. The sand filter is at the waterworks. The water passing through a waterworks comes from reservoirs, lakes and rivers. The water from these places has small solid particles suspended in it. The sand filter removes the solid particles. The holes in the filters are not small enough to trap germs called bacteria. These are killed by adding chlorine gas to the water before it leaves the waterworks.