Once a tiki mug has been hairline cracked, does it continue to spread? - General Tiki

C coruscate Member Petaluma & Point Reyes Station Joined: Nov 26, 2007 Posts: 149	C coruscate Posted posted on Wed, Sep 9, 2009 2:53 PM FOR YOUR TMI files: This is a problem with all ceramics, but more so with "low fire." Many Tiki mugs are made with Low Fire materials fired at a lower cone. Fuel is expensive, so if you can make a glazed object at a lower temperature, then you save lots of money and time heating up and cooling a kiln quickly. (e.g.cone 06 to cone 1, a cone is a "tattle tale" shaped in a cone about the size of a golf T that bends when exposed to and saturated with heat) When making functional pottery, the glaze and the clay need to fit. Clay and glaze are two different materials being heated and they expand and contract at different rates as they approach being melted to cooling again. If they do not fit, e.g. the glaze and clay expand and shrink differently, then it is like clothing not fitting and you end up with the clay equivalent of Chris Farley as a fat man in a little coat. This can result in catastrophic shattering, like an earthquake releasing stress, or in crazing and de-laminating over several decades as the stress is relieved in slow motion. It is hard to believe that something that helps reduce so much stress can be "tightly wound," but that's what I think is going on. If the goal is to make cheap mugs to give away or that break easily and forces the restaurant to re-order another case, then Low Fire is the way to go. It also allows more color control and a broader palette of colored minerals that don't "burn out" like they do in High Fire. These "cheap mugs" are often the mugs we adore. In "pottery" there are four general states of the ceramic object during production: Greenware (further subdivided into wet, leather hard, and bone dry), Bisqued (basically toasted, it is cooked at a temperature high enough to drive all the physical water molecules and chemically bonded water out, making it optimum for glazing), Glazed ( a fine layer of powdered minerals and silica is coated on the bisqued mug, often by spraying or dipping the mineral in a slurry the consistency of milk and then drying), and Fired (which is a huge range of finish products from the point where you've just barely melted the silica to make a thin glass coating or all the way to vitreous where you've completely melted the silica throughout the clay body in addition to the glaze). Because vitreous pottery is completely melted (but stopped before it pools on the shelf) it is impervious to liquids, is very durable and the glaze tends to fit really well (or it shatters before or while you open the kiln) making it ideal for pottery but expensive to create because of fuel and labor cost. A few tiki mugs are made this way, but are rarer. Most tiki mugs are low fired, where the porous bisqued mug is heated to just past the point where the coating of minerals on the clay surface are fluxed into glass Because it isn't heated to the extremes of vitreous ceramics, low fire mugs don't tend shatter from stress, but can be under tensions like a tightly topped drum, and are more prone to breaking. The glaze makes the surface impervious to liquids, but the unglazed portions are still absorbent and moisture can enter from the air, the beverage or immersed in the sink. The moisture content is constantly equilabrating with the surrounding air. By analogy, just as a sponge shrinks and expands in volume as it is soaked and dried, so does your mug. There are unglazed portions of the mug, creating uneven pathways for moisture and therefore uneven volume. Crazing and cracks create more pathways for moisture. Occasionally this extra bit of stress may be the last straw for the mug (pardon the pun). Retiring the mug from use may be the only choice for it. Air conditioners often dry the air in really humid environments, reducing the moisture swings. Cyanoacrylates (CA, aka super glue) as someone suggested, when placed along the hairline may help bond it at the crack, but it also acts like a caulking. You can theoretically reduce the problems caused by moisture on low fired ceramics by controlling the humidity and temperature. Enclosed shelves with desiccant or moving to Palm Springs can possibly help. I hope that gives you some extra ways to think about preserving your mugs for longer enjoyment. Coruscate

FOR YOUR TMI files:

This is a problem with all ceramics, but more so with "low fire."

Many Tiki mugs are made with Low Fire materials fired at a lower cone.

Fuel is expensive, so if you can make a glazed object at a lower temperature, then you save lots of money and time heating up and cooling a kiln quickly. (e.g.cone 06 to cone 1, a cone is a "tattle tale" shaped in a cone about the size of a golf T that bends when exposed to and saturated with heat)

When making functional pottery, the glaze and the clay need to fit.

Clay and glaze are two different materials being heated and they expand and contract at different rates as they approach being melted to cooling again.

If they do not fit, e.g. the glaze and clay expand and shrink differently, then it is like clothing not fitting and you end up with the clay equivalent of Chris Farley as a fat man in a little coat.

This can result in catastrophic shattering, like an earthquake releasing stress, or in crazing and de-laminating over several decades as the stress is relieved in slow motion.

It is hard to believe that something that helps reduce so much stress can be "tightly wound," but that's what I think is going on.

If the goal is to make cheap mugs to give away or that break easily and forces the restaurant to re-order another case, then Low Fire is the way to go. It also allows more color control and a broader palette of colored minerals that don't "burn out" like they do in High Fire.

These "cheap mugs" are often the mugs we adore.

In "pottery" there are four general states of the ceramic object during production:

Greenware (further subdivided into wet, leather hard, and bone dry),
Bisqued (basically toasted, it is cooked at a temperature high enough to drive all the physical water molecules and chemically bonded water out, making it optimum for glazing),
Glazed ( a fine layer of powdered minerals and silica is coated on the bisqued mug, often by spraying or dipping the mineral in a slurry the consistency of milk and then drying), and
Fired (which is a huge range of finish products from the point where you've just barely melted the silica to make a thin glass coating or all the way to vitreous where you've completely melted the silica throughout the clay body in addition to the glaze).

Because vitreous pottery is completely melted (but stopped before it pools on the shelf) it is impervious to liquids, is very durable and the glaze tends to fit really well (or it shatters before or while you open the kiln) making it ideal for pottery but expensive to create because of fuel and labor cost. A few tiki mugs are made this way, but are rarer.

Most tiki mugs are low fired, where the porous bisqued mug is heated to just past the point where the coating of minerals on the clay surface are fluxed into glass

Because it isn't heated to the extremes of vitreous ceramics, low fire mugs don't tend shatter from stress, but can be under tensions like a tightly topped drum, and are more prone to breaking.

The glaze makes the surface impervious to liquids, but the unglazed portions are still absorbent and moisture can enter from the air, the beverage or immersed in the sink.

The moisture content is constantly equilabrating with the surrounding air.

By analogy, just as a sponge shrinks and expands in volume as it is soaked and dried, so does your mug.

There are unglazed portions of the mug, creating uneven pathways for moisture and therefore uneven volume.

Crazing and cracks create more pathways for moisture.

Occasionally this extra bit of stress may be the last straw for the mug (pardon the pun).

Retiring the mug from use may be the only choice for it.

Air conditioners often dry the air in really humid environments, reducing the moisture swings.

Cyanoacrylates (CA, aka super glue) as someone suggested, when placed along the hairline may help bond it at the crack, but it also acts like a caulking.

You can theoretically reduce the problems caused by moisture on low fired ceramics by controlling the humidity and temperature.

Enclosed shelves with desiccant or moving to Palm Springs can possibly help.

I hope that gives you some extra ways to think about preserving your mugs for longer enjoyment.

Coruscate

Post #482387 by coruscate on Wed, Sep 9, 2009 2:53 PM