After a while, a ball point pen doesn't write very well anymore. It will write for a little distance, then leave a gap, then maybe write in little streaks, then maybe write properly again. It seems to be worse with older pens, but I have observed this with new pens right out of the box too.
Experiments I have done:
- Take the cartridge out and look at the amount of ink. There is still plenty.
- Inspect the ball with a jewler's loupe, no obvious damage, everything looks smooth and clean.
- Stored new pens unused tip-down to eliminate gravity slowly pulling the ink from the ball and leaving a air pocket. Some of the pens exhibit the symptom even when used the first time with the cap never removed before and stored this way for a year.
- Stuck a wire in the open end of the ink reservoir to see if maybe the end dried to a hard plug so that new ink couldn't move down as it was removed from the reservoir by writing. I have never found anything hard, and observed the same symptoms even after "stirring" the top of the reservoir with a wire a little.
- When a pen stops writing, shake it hard, like resetting a fever thermometer. That seems to help for a brief while, but so does just waiting a few seconds, so I'm not sure the shaking is relevant.
- Stored a pen ball-down in a glass of water overnight. The thought was if the ink just above the ball had dried, maybe this would re-constitute it. Some ink clearly dissolved in the water since it was colored, but once the pen was started up again there was no apparent change to the symptoms.
- Cold seems to exaggerate the symptoms, but warming to body temperature doesn't fix them.
This is not just a single pen or a single model. I have bunch of different pens of different models that do this. I'm curious, what causes this?
I have done some more experimenting, and it seems Emile Jetzer was right. The cause seems to be that the ink is so viscous that new ink doesn't flow down to replace what is removed via the ball fast enough. Two experiments support this hypothesis:
- A pen will write again after a while by just letting it sit ball-down, but the time is significantly decreased when you shake it, like you would resetting a fever thermometer.
- Some stick pens are sealed except for a small air hole at the top. Putting lips around the top of the pen and applying pressure as if you were trying to blow into it resets the writing action quickly. Even better, I can write with such pens much longer than they would normally go by holding my mouth over the top and applying constant air pressure.
So, I think the mystery is solved. Probably ink in the reservoir dries out slowly over time by losing water vapor from the top. That makes the entire ink more visous, which explains why old but unused pens also exhibit this symptom.
The next experiment is to take such a pen and add a little water at the top of the ink reservoir, then let it stand for a week and see what difference that makes.