A replacement projector bulb should be handled by the porcelain base to prevent ____ from contacting the bulb surface.

Handling a replacement projector bulb by the porcelain base is crucial mainly to prevent skin oil from contacting the bulb surface. When skin oils are transferred to the bulb, they can create hot spots when the bulb is in operation. These hot spots may lead to premature bulb failure or even cause the bulb to shatter due to uneven heating. By focusing on holding the bulb by the porcelain base, which is typically a part of the structure that does not come into contact with the glass, the integrity of the bulb is maintained.

While factors such as dust, water, and heat might have their own implications, they do not directly relate to the immediate risks associated with skin oils. Dust can be cleaned off; water is usually not a concern if the bulb is properly installed; and heat is a natural part of how bulbs function. However, oils from fingers can severely compromise the operational lifespan and safety of the bulb, which highlights the importance of this handling technique.