An ink reservoir is what makes a fountains pen a fountain pen.
They come with a choice of two types of reservoirs: removable – being cartridges and converters or permanent – include eyedropper, piston, vacuum, and sac. Each have their own unique pros and cons.
These are pre filled with ink and are disposable. They are relatively expensive compared to bottles of ink and the choice of colors is limited. Yes you can cheat (assuming you have a syringe) and want to go through the challenge of self filling the cartridge with the ink of your choice.
These are the easiest to replaced, simply unscrew the section from the barrel, pull the now empty cartridge out and insert a new one. They come in various lengths so make sure the one you choose fits your pen.
Converters are essentially replaceable cartridges and we can fill with whatever ink (never India ink) we choose. Some converters screw into place, while some push in like a cartridge. Like cartridges, they also come in a variety of lengths so make sure the one you choose fits.
Filling a converter is accomplished in a couple fashions. My preferred method, install the converter, engage the fill mechanism to depress the plunger. Dip the pen into the ink, work the mechanism again. As the plunger retracts it creates a vacuum and draws ink into the converter via the nib. It can be a messy process as you have to clean the nib afterwards but the pen is ready to use. The alternative process is just like the previous except before the converter is installed you stick the open end of the converter into the ink directly then engage the mechanism and draw in the ink. Lastly, using a syringe is another option and makes a clean alternative to filling a converter.
Essentially, a fixed converter that uses the inside of the barrel as the reservoir. Twist the filling mechanism left or right to transition the plunger in the barrel. To fill, place the nib in ink and rotate the mechanism clockwise. This retracts the plunger and draws ink into the barrel. This type of reservoir is difficult to clean. On a side note, this filling system was patented in 1923.
A Parker proprietary design mechanism. The inside of the barrel is the ink reservoir, a plunger mechanism with a diaphragm is locate at the opposite end from the section. When the plunger on the mechanism is depressed it expands the diaphragm, expelling air. When released the diaphragm retracts and ink is drawn into the pen. To fill the pen simply insert the nib into a bottle of ink, press the plunger and release, ink fills the pen. This type of reservoir is tiring to clean. On a side note, this filling system was patented in 1933.
Ink sacs are mostly found in vintage pens and the occasional contemporary. The sac is attached to the section and when depressed by a filling mechanism, the air or ink is expelled from the sac. When the mechanism is released the sac expands and draws in ink. Common filling mechanisms associated with a sac reservoir include a Lever, Crescent, Areometric, Button, and Leverless. This type of reservoir does not clean well.
Makes use of the entire hollowed area of the barrel as a reservoir. The section is removed thus providing access to the reservoir. The threads of the section are usually sealed with a silicon grease or an o-ring to keep the pen air tight. To fill, remove the section then using an eyedropper extract ink from a bottle and squirt it into the barrel. Filling the pen requires care as one slip, the pen falls and ink flows everywhere. It is by far the easiest to clean and provides the highest capacity reservoir.