Abrasives
“Will the captain of each group write on the board the word about which the group is to report today?” asked Miss Dean.
“What has the ‘chalk’ group to tell us?”
Peter was captain of this group. He rose and said:
“We found out that chalk comes from the shells of tiny animals that live in the sea. When the animal dies the shell is left and it sinks down through the water to the sea bottom, where it makes ooze, or mud, the grains of which are very fine. There are billions and billions of these animals in sea water, and in some places their shells are found in great heaps. One place is along the southeast coast of England, where there are high cliffs of chalk. Once these were below the water, but something happened to push them far out above the sea.”
“There must be a great many of these shells in a big chalk cliff,” said Miss Dean. “How large are these animals that make chalk?”
“Not large at all,” said Peter. “Some of them are so small that they can hardly be seen except through a microscope. Here is a picture of them as they seem when we look at them through a microscope. Here the little animal is magnified to be about eighteen times as long and broad as it really is.”
“Why, they are beautiful,” said Grace. “Look at the tiny needles they seem to have all over their bodies. Are they called chalk animals?”
“No,” said Miss Dean. “The very tiniest creatures sometimes have the longest names, and the name of these chalk-forming animals is Foraminifera.”
“Fo-ram-i-nif’-er-a,” said the children, slowly, after Miss Dean. “What does it mean?”
“The word means that these animals carry shells which are pierced or bored with tiny holes,” said Miss Dean.
“These spines which look like needles are part of the shell. Both the shell and the spines are made mostly of limestone. But what did you find out about the uses of chalk?” asked Miss Dean.
“We found out,” said Peter, “that chalk is used for fine polishing. It is ground to powder, and then put in water. The finest chalk floats on the water, and that is the kind found in some tooth pastes and powders. Usually, though, a special kind of chalk is prepared chemically for tooth pastes or powders. Whiting is another name for chalk. It is often used for silver polishes.”
“Does the chalk which we use at the blackboard come from these little animals?” asked Tom.
“Not entirely,” said Miss Dean. “We used to use pure chalk. Now, however, it is often a mixture of chalk and other things.”
“In England thousands of tons of chalk are dug out of the chalk cliffs every year,” went on Peter. “Beds of chalk are also found in our own country, in Kansas, and in Texas.”
“Then those places must once have been under water,” said George.
“Don’t you remember, when we talked about hard and soft water, that I told you about the great sea which covered the middle part of our country long ago?” asked Jimmy. “So some of the lime which is found in those states would come from shells of chalk animals.”
“Miss Dean, I’m glad my question is answered,” said Mary.
“What did the group find out about diatoms?” asked Miss Dean.
Grace answered. “We found that diatoms are tiny plants with shells, something like the chalk-forming animals, except that their shells have silica instead of lime and are round with no spines. Here they are, in the same magazine with the chalk-forming animals. They are found in water almost everywhere, in oceans and rivers and lakes. They often cling in slimy masses to wood or stones under water. Some of the land near the Mississippi River is almost all made of the shells of diatoms mixed with sand, sometimes down hundreds of feet.”
“Is the Mississippi River every giving diatoms to the land along its banks in these days?” asked Miss Dean.
No one answered for a moment, and then Jimmy said:
“The Mississippi River has big floods sometimes, when it spreads out over the land on each side. I should think it would carry diatoms to the land when it does that.”
“I am sure it does,” said Miss Dean, “so you see our abrasives are being made and brought within our reach right along. But how are diatoms made into abrasives?”
“The shells may be washed free from sand,” answered Grace, “and as they are so tiny and round I should think they would make very fine abrasives.”
“Miss Dean, Grace said the shells of diatoms were made of silica. What is silica?” asked Tom.
“Silica is a hard, flinty mineral which is found almost everywhere in the soil and rocks. I think we shall hear about silica from the group which is to report on quartz. Silica is harder than chalk, so when found in the shells of diatoms it makes a harder or firmer abrasive than chalk. Some of these finer powders for general use, which showed rounded shapes under the microscope, may contain diatom shells.”
“And now let us hear about lava,” said Miss Dean.
“We thought it was great fun to find out about lava,” said Jimmy for his group, “because we like to read about volcanoes. You see volcanoes have openings called craters, and melted rock pours out of the craters when the volcanoes are in eruption. The melted rock is called lava, and it often spreads out over miles of country. A great deal of lava is found in many places out west, in the Rocky Mountains.”
“Is lava used in scouring powders?” asked Miss Dean.
“Yes, but more often in the form of volcanic ash. With the beds of solid lava are found great beds of lava ash. Pumice, another kind of lava, is also used a great deal.”
“’Pumice’ is our word,” said George. “Don’t tell us about that.”
“I won’t,” said Jimmy, “only I wanted to say that powdered lava, or ashes, and solid lava are found together in Nebraska and Kansas and Idaho and several other states in the West. Volcanic ash is used for scourers, and I think it is the kind which seems to have flat, thin pieces under the microscope.”
“And pumice,” said George, as Miss Dean nodded to him to begin, “is lava which has been blown full of bubbles by steam or other gases bursting up through it while it is cooling. It is like froth or soapsuds turned to stone.
“Pumice, or pumice stone,” he went on, “is so porous, or full of little bubbles, that sometimes pieces of it will float on water. It is ground and sifted for use as an abrasive in scourers and metal polishes, and it looks about the same as volcanic ash, I think. The book said it had fine, flaky particles after it was ground. I brought a piece of pumice stone to show the class. Here it is.”
“Oh, yes, one of our group brought this piece of lava,” said Jimmy.
The lava and the pumice went from hand to hand, and John said:
“My! I wish I could have been there when the lava was being blown full of bubbles like this.”
“There must be lots of volcanoes in the Rocky Mountains,” said Mary.
“Not now,” said Miss Dean. “The lava and ashes and pumice stone which are found out in Nebraska and the other states Jimmy mentioned were made ages ago, probably about the time that the mountains were rising from the sea. At that time the boiling lava probably ran into the water and turned it into steam, and the steam in turn pushed its way up through the lava, forcing holes through the cooling mass.”
“We also bring pumice to the United States from the country near Mount Etna, a volcano in Sicily,” went on George.
“Don’t you think scourers become interesting,” asked Miss Dean, “when you think of the abrasives we use being formed by these great forces in the earth and by billions of creatures in the sea?”
“But why do we go so far for abrasives?” asked John. “Why not grind up sand?”
“That’s what we talk about,” said Mary, and she showed a bottle full of fine beach sand. “This is mostly quartz, or silica, and, as Miss Dean said, silica is almost everywhere in rocks and in the earth under our feet. We may take sand like this and grind and sift it for use in scourers. Sometimes it is ground quite coarse for hard scouring, and sometimes it is ground into very fine particles. One kind is ground so fine that it is called silica flour, and it may be used for polishing jewelry. Quartz is quite scratchy when it is coarse, as the little flinty pieces are like grains of sand, and have sharp sides and points. We saw some, I think, in our coarser scourers. A good grade of silica is found in Illinois and in Maryland.”
”This has been interesting to me,” said Miss Dean, “and I’m glad we began to study scourers. Next time we will take up our last question, which asks what other things may be in scourers. Then I think we shall be able to make some scouring soap for you to take home.”
This chapter suggest several things which will be fun to look up. For instance:
Look in your geography for the map of Europe and find England. Perhaps you can find pictures of England's chalk cliffs.
Find Dover on your map of England. Some famous chalk cliffs are along the coast here. From what body of water would you have a good view of them? Describe how you think they would look.
Why was the name Albion given to England many hundred years ago? (The Latin word for white is albus.) Perhaps Julius Caesar gave the name. Why might he have done so?
For what is chalk used besides for abrasives and blackboard crayon?
Have you any books or magazines which have pictures of chalk-forming animals or foraminifera? And diatoms?
Follow along the Mississippi River on your map of the United States, and name the states which might have diatoms in their soil.
Find the states which Jimmy said had great beds of volcanic ash and lava and pumice.
Are there any volcanoes in our country which are sending out lava and making pumice now? Are there in any other countries? What have you heard about a volcano in Hawaii?
A good grade of pumice comes from the Lipari Islands. Find them, near the toe of Italy's boot. What would be an explanation for the finding of pumice there?
Could you make a collection of chalk, diatoms, lava, pumice, and quartz? The chalk, in powdered form, and pumice you may buy at the drug store or at a department store. Perhaps you have pumice in your bathroom which is used for scrubbing stains from your fingers. And where do you think you can get diatoms? Go to a paint store and ask for rottenstone. It will have diatoms in it, perhaps miced with fine sand. It is used for rubbing down furniture to make a smooth surface. As for lava, perhaps someone in the class has a piece at home. And I think you know where you can find some nice, clean quartz.
Internet Links to Help you on your Way
England
An Outline Map of England for You to fill in
Map of England
Dover
Dover Historical Museum
Dover, Gateway to Europe
Albion
Albion, Ancient name of England
Foraminifera
Types and Images
The Mighty Mississippi River
The Mississippi Delta Queen
Italy
An Outline Map of Italy for You to fill in
Map of Italy
Where would you like to go next? 
