334 MARKET DAIRYING 
17. Do not fail to empty the boiler every week or two 
and refill with fresh water. 
18. Have your steam gauge tested at least twice a 
year. 
BOILER INCRUSTATION. 
In all boilers after a period of use, there is deposited 
upon the parts below the water level a scale or sediment 
known as boiler incrustation. 
Cause of Scale. The formation of scale is due to 
the impurities contained in the feed water. When impure 
water is fed into the boiler the impurity first manifests 
itself in the form of scum on top of the boiling water. 
The heavier particles of the scum slowly unite and sink 
to the bottom where they first appear as mud. By con- 
tinued exposure to high temperature, this mud gradually 
forms into a hard impervious scale which usually con- 
sists largely of lime. 
Objection to Scale. 1. The excessive formation of 
boiler scale is the immediate cause of most boiler explo- 
sions. The scale acts as a non-conductor of heat, so that 
in cases where the capacity of the boiler is severely taxed, 
the metal becomes overheated, thus materially weakening 
it. The scale is, therefore, not only dangerous, but by 
overheating the metal, also materially shortens the life 
of the boiler. 2. Another most serious objection to scale 
is its wastefulness of fuel. This becomes evident when 
we note that the heat before reaching the water must first 
be conducted through a non-conducting layer of incrusta- 
tion. 
Prevention of Scale. Since nearly all water used for 
boilers is more or less impure, it is evident that to prevent 
scale, boilers must receive frequent cleaning. How often 
