英文科学读本 第一册·Lesson 13 Clay—Its Uses Ⅱ

Lesson 13 Clay—Its Uses Ⅱ

The children were called away before they could finish their chat about clay. They had to leave it till some other time.

The next evening Fred began by making a little rough basin with his piece of clay, just as he had seen teacher do. He then filled the little basin with water.

I know what that is for. said Norah. "You want to prove that the moist clay is not porous."

Fred then put a piece of dry brick into a basin, and filled the basin with water to the brim. They waited for a minute or two and then he said, "Look, Norah, some of the water has gone. What has become of it? It has not been spilt on the floor."

I suppose it has gone into the brick, said Norah.

That's right, said Will. "Now what does this show?"

If the brick has sucked up the water, it must be porous. said Norah.

So it is, said Fred. "The baking made the clay porous."

Now listen, he added. "We want our bricks to be porous, so that any water that gets into them may drain out. But our cups and saucers, jugs and basins, and most of our earthenware things have to hold water. They must not be porous, or the water would run away."

But you told me they were all porous after they had been baked. said Norah.

Don't you be in a hurry, little sister, said Fred. "Here is a flowerpot, and here is a teacup. The teacup was as rough as the flowerpot at first, but it is smooth and shiny like glass now. We say it is glazed. This glaze is not porous, and liquids will not pass through it. After the cup is baked it is dipped into the glazing mixture and baked again. When it comes out of the oven this time it is smooth, shiny, and not porous, just as you see it now."

What clever boys you are, said Norah. "You are very good to tell me all these fine things."

SUMMARY

Vessels made of baked clay are porous. Bricks and flowerpots are porous. Cups and saucers, plates and dishes, basins and jugs are glazed, to make them smooth and shiny like glass. After this they are not porous.