Unlike ballpoint pens, fountain pens contain a lot of detail and expensive craftsmanship in the nib. Thus, nearly all fountain pens are designed to be refillable. A number of different refill mechanisms exist, including piston fillers, sac fillers and lever fillers, but most low end fountain pens use ink cartridges.

Fountain pen ink cartridges date back to the 1950s. Some early models were made out of glass, but various plastics quickly became the material of choice. Some manufacturers experimented with refillable cartridges, but these were messy and unpopular; disposable cartridges quickly became the norm.

Advantages and Disadvantages

Cartridges are considered by some to be more convenient than bottled ink. This depends a lot upon the pen's filling mechanism — whilst a good cartridge pen is less prone to mess than a bad bottle filler, a modern piston filler is very clean. Cartridges may also be easier to carry around than some ink bottles.

There are downsides to cartridges too. They are typically more expensive than bottled ink and are available in fewer colours and styles. The ink flow from cartridges tends to be slower, which can be a problem with some nibs. Also, pens designed to take cartridges carry less ink than a dedicated sac filler of similar dimensions.

Sizes and Manufacturers

Cartridges come in different sizes. Some pen manufacturers, including Aurora, Lamy, Parker and Sheaffer, use proprietary cartridge sizes. Others, including Caran d'Ache, Conway Stewart, Montblanc, Pelikan, Rotring, Yard-o-Led, more recent Waterman pens and most cheap unbranded European and Chinese pens, use the international cartridge size.

The international cartridge is not a specific documented standard. Nor does it have an agreed-upon name — it is also called European (usually by American pen manufacturers), Standard and Universal. There are two sizes of international cartridge, sometimes called short (38mm in length) and long (72mm in length). Some pens will take both long and short cartridges, and some will only take short cartridges. Some pens are also designed to accept a spare second short cartridge stored in the barrel.

The international format is also used by many specialist ink manufacturers, including J. Herbin, Private Reserve and Diamine, for some of their more popular colours. Although this increases the amount of choice available to cartridge pen users somewhat, it still does not match the variety available in bottled form.

Note that Waterman long cartridges, which are sold as "Large Size Standard Cartridges", will not work in many non-Waterman pens; non-Waterman large international cartridges will, however, work in modern Waterman pens.

Using an incorrectly sized cartridge (for example, attempting to use an international cartridge in a Parker) can lead to leaks. In some cases it can also lead to the pen being damaged. Most manufacturers will not honour a warranty claim if the damage has been caused by using the wrong type of cartridge.

Styles

Close inspection of ink cartridges will show that there are two popular styles of opening. Some manufacturers simply use a very thin piece of plastic, which is punched through by a special connector inside the pen. Others use a small ball (ever so slightly wider than the cartridge opening) which is initially lightly glued to the mouth of the cartridge; when inserted for the first time, the ball is pushed back into the cartridge's body.

In theory, the ball style is slightly less prone to making a mess when the cartridge is removed and held upside down — the ball should fall and block the opening, rather than relying upon surface tension to hold the ink in place. The ball style also requires less effort to insert, reducing the chances of a messy accident. In practice there is little difference between the styles.

Most ink cartridges are sold in small plastic or cardboard boxes known as cards. However, J. Herbin cartridges come in attractive brushed metal tins.

Converters

Recently, fountain pen manufacturers have started to use cartridges even in higher end pens — many customers prefer the convenience of cartridges despite the associated disadvantages. To please the more serious pen users they include special converters which allow bottled ink to be used. These converters are shaped like an ink cartridge, but include a piston which allows the cartridge to be filled up. This does not provide the increased capacity of non-cartridge pens, but it does allow far more flexibility when choosing an ink.

A number of generic converters for international fountain pens are available if the manufacturer does not provide one. These will work in most pens which can take long cartridges; pens which can only take short cartridges usually do not have enough room in the barrel for a converter.