Why are there air bubbles in my tubes of watercolor paint?

Question

I'm opening new paint tubes for watercolors, and about 50% of the time, the paint starts gushing out slowly. I then hear a few pops. This would be okay if it only happens once when I open the bottle. I opened the black paint tube yesterday, no gushing whatsoever. Today I open it, and the entire inside of the lid and screw threads on the tube are completely submerged in black paint, and there's a worm of paint slowly making its way out.

Why does this happen, and is there any way to prevent or stop it?

Answer 1

Initially, the gushing of the paint is due to the small amount of water that generally is part of the product's formula. This quote from handprint.com explains the reason and effect:

Paints are manufactured with excess water in the vehicle, as this reduces the viscosity of the vehicle and decreases the amount of time (labor) and electrical energy necessary to mill the paint. This water mostly lost through evaporation during milling, but also after milling when the paint is left to sit and age or stabilize. Some pigments or fillers absorb water very slowly, causing them to expand: these are the paints that "explode" or squirt from the tube when it is first opened, because they were not aged adequately before packaging.

The amount of water involved varies from pigment to pigment (some need more grinding or more aging), and brand to brand (due to production techniques).

I would ascribe the air bubbles to an unsophisticated production process, though, in which the tube is not completely filled or improperly sealed.
Oxygen could have formed due to ongoing chemical processes inside the tube, but that should also have been considered before packaging.

As for subsequent - and erratic - eruptions, apart from pressing the tube by hand when opening it, which can be avoided, I think this can only be ascribed to changes in atmospheric conditions: temperature or air pressure, of which the first seems the most likely culprit: a rise in temperature would increase the pressure inside the tube.

To be thorough: I don't suppose the tube was stored somewhere where something could have been (temporarily) put on top of it, or where someone could have 'played' with it? That would make for an obvious cause as well :)

Bottom line: always open tubes above your palette so whatever comes out can be used.

Answer 2

This happens with many semi-liquid products that are packed in tubes, not just paint. I notice it with some cosmetics and skin creams as well. It's nearly impossible to open a tube without compressing it, which pressurizes it and causes the contents to come spurting out. One solution is to store the tube with the cap pointing up. This way you usually get an air pocket inside the cap, so when you open the tube what comes gushing out is air rather than paint. It also helps to grip the tube near the cap rather than further down the body. This reduces the amount that you compress the tube, so there's less pressure when you open it.