Since the 1890's, only a "safety fountain pen" has safely used many types of bonded inks that would ordinarily be harmful to other fountain pens, such as
A safety pen is safe to shake when capped, to fly with on an airplane, and can survive other pressure variations without leaking when well capped. In a safety fountain pen, the nib and feed are bathed in ink completely, unlike any other kind of fountain pen. This is why they can use inks other pens cannot.
The Noodler's Ink Boston Safety pen is crafted from ebonite/hard rubber with o-ring seals and can use a far wider variety of writing fluids than any other fountain pen types. All Boston Safety pens will now come with a metal core to prevent the inner parts of the pen from snapping. This should increase the durability and longevity of this pen tenfold.
The fitted stainless steel Noodler's Ink flex nib benefits from this pen design to keep the flex writing with adequate flow.
To see how to fill and use the Noodler's Boston Safety, please watch the Noodler's Ink instruction video here.
Noodler's Ink Boston Safety Fountain Pen Specifications