Clay, silt, salt, mica or organic matter in the water will weaken concrete and ferro-cement, as will certain invisible chemicals. Water that is fit for drinking is usually fit for mixing cement mortar. The quality of an unknown water can be tested by comparing it with water known to be good. This is best done by using the known water (such as drinking water), making three cakes of cement paste, each approximately 20 mm thick and 60 mm in diameter. At the same time make three identical cakes using the unknown water, and compare setting and hardening times of the two. types. The cement paste is prepared like the nil coat, adding cement in a small container with water while constantly stirring the mixture. Cement should not be added fast to avoid clotting or the development of lumps. To achieve equal test cakes, a glass can be used for shaping. All cakes must be of equal size and shape. A chart should be prepared to record the test results (see Fig. 3.3). Mark on your chart that the sample has set when you can no longer make an indentation with your finger tip. Mark the samples with A for drinking water mixture and B for unknown quality. Test for hardening by marking whether or not you can scratch the sample with your fingernail (see Fig. 3.3). The samples must be stored in the shade. If the chart shows that both samples are nearly equal, the water of unknown quality can be used for mixing cement plaster.