When a steam boiler is installed, there is an oil residue from the manufacturing, oils on the 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 that 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's 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 hole. The steam moving at a higher velocity reaches the boiler supply tapping and starts squeezing into the openings, increasing its velocity even more.
Now we start producing a venturi effect as the steam rushes to a lower pressure zone, the system piping 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 more slowly and gives up the latent heat much quickly, causing the steam to turn into condensate. This water must return to the boiler, reheated, and turned back into steam. The boiler must use 970 BTUs to recreate this steam.
The water leaving and dropping back into the boiler is called surging and can cause the water level to bounce 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. If there is enough water in the boiler, an automatic water feeder will add more water. When all this condensate and added water gets back into the boiler, it is now overfilled or flooded.

When the water level in the gauge glass bounces more than 1/2" or the water level drops to the bottom of the glass or even disappears completely while steaming, the boiler needs to be skimmed.