When a steam boiler is installed there is an oil residue from the manufacturing, oils on iron pipe, oils from cutting and threading the black iron pipe and possible oil from fluxes and thread sealant.
All steam carrying pipes should be iron pipes and the piping below the water line of the boiler can be iron or copper. The reason for this is twofold. When copper steam carrying pipes are used this allows the steam to leach copper molecules from the copper tubing. The copper molecules will end up back in the boiler which sets up a galvanic reaction and may shorten the life of the boiler. The second reason is the expansion rate is a lot and all copper joints are brazed, or silver soldered which has no play or give. The near boiler piping calls for swing joints which absorb the expansion by rotating slightly when heated and cooled and cannot be carried out without threaded fittings.
The water is then put in the boiler and the switch is turned on. As the water continues to heat up, the oil will gradually float to the water surface. The oil slick slows down the escape of steam into the boiler steam chest and piping. The tiny steam bubbles will join forces and become larger and larger. When the steam bubble gets large enough to break through the oil it will be moving at a higher velocity. Once a hole is blown through the oil slick all created steam bubbles will take the path of least resistance and use the same whole. The steam moving at a higher velocity finds the boiler supply tapping's and starts squeezing into the openings and the velocity increases even more.
Now we start producing a venturi effect as the steam rushes to a lower pressure zone, the system piping, it sucks water from the boiler up into the pipe with it. The water level in the sight glass starts to drop. Most of the water is too heavy to continue up the pipe and drops back into the boiler. The water level in the sight glass goes up. We now have wet steam which moves slower and gives up the latent heat much quicker and causes the steam to turn into condensate. This water must return to the boiler, reheated, and turned back into steam. The boiler must use 970 btu's to recreate this steam.
The water leaving and dropping back into the boiler is called surging and can cause the water level bouncing up and down more than about 1/2".
When the water bounces an inch or more it is considered severe surging. Some of the water gets into the system and runs down the main pipe condensing steam to water along its way. Enough water may leave the boiler if there is an automatic water feeder it will add more water. When all this condensate and added water gets back into the boiler it is now overfilled or flooded.