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

Lately with all of the hot weather and humidity, I've been hearing strange ping noises every so often, coming from my tiki mug shelf. I have two vintage mugs with known hairline splits running down the sides, but I was wondering if this was normal to hear, or does weather play tricks on the ceramics?

I remember a similar sound a few years back, except much louder ceramic crack noise when my Shag 40th tiki room anniversary decided to split open, right after receiving it.(thankfully it was replaced by Disneyland/Holden). Apparently it either caused by the clay mold not being dry enough, and adding a dual two tone glaze, which was fired, trapping moisture within the mug... or as the glaze was drying, it shrinks just so slightly as it hardens, that the inside glaze(orange) dried at a different rate than the outside glaze(brown) did, causing stress on the mug. It was loud enough, that the crack(ping) noise woke me up at 7am. I discovered the clean 4 inch crack a few hours later.

I don't know, but it has me a bit nervous. It's not like I can easily remove each of my mugs to inspect them, due to having used museum putty on all of the bases of them for the, "just in case of an earthquake" safety precaution. By the way, I highly recommend the stuff. It is museum wax, and bonds well!

Anway, has anyone else heard their tiki mug shelves creak/ping/pop, and is it a normal happening?

Changes in humidity and temperature can stress a hairline crack in a ceramic vessel and cause it to break 'spontaneously.' Where do you have your mugs stored/displayed?

[ Edited by: Chip and Andy 2009-09-08 10:36 ]

the mugs are on a multi tiered wall shelf in my bedroom. I hope it is just the two already split/cracked vintage mugs(I have two other identical replacements in near mint condition, so I'm not too worried if it is the two that are already damaged, and possibly cracking more)

turn on the AC son.
keep them from getting too hot.

Jeff(btd)

When my vintage mugs have cracks in them I dont handle them at all. In fact I put them up somewhere away from the sunlight and the air conditioner.

I wish there was a way to seal the crack once it begins.

I offer no advice for ya Tom, just nod my head in agreement to having the same problem.

I actually hear that all the time, and I'm in Hawaii, so I assume its the humidity.

I haven't noticed any actual cracks in mugs, but have noticed that certain new mugs have that vintage crazed look happening to the glaze.

Happens mostly to the ones with thick, glossy glaze.

What you might try to stop the crack from spreading is get some super thin CA glue (aka super glue), most hobby stores will have this. The super thin glue will wick into the crack and if done right will not be visible at all.
Don't try to apply it from the bottle, put some in a small cap (or similar thing) then use a large pin to get a drop on the end, touch it to the crack and it'll go right in to the crack.

[ Edited by: Lightning Steve 2009-09-08 21:49 ]

I wish I hadn't read this thread.. your scaring me now....
I hate crack heads...:wink:
aloha

Damn! I should have checked today's weather report.

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:

1. Greenware (further subdivided into wet, leather hard, and bone dry),
2. 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),
3. 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
4. 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

Even though Tom asked the question originaly I appreciate all the info. I am going to attempt the super glue trick tonight on a mug that has a hairline.

Thanks for posting it.

Some really good information, and thanks to all who chimed in! Coruscate basically summed up what my brain was thinking in alot of areas covered in the post.

Now, with all of the mugs being manufactured in China, and China being known for its ceramics that have lasted literally ages and Dynastys...Are they using low fire, or high fire, or does it depend on the company ,like anywhere else? Also, can the grade/type of silica used for glaze have an impact as to how long the mug will hold up? is their even a silica grade system for glazes? Can there be inferior silica to cause premature crazing/cracking?

Sorry for all of the TMI questions, but these are kind of curious to me, and I'd like to know, for those useless information conversations that tend to pop up! haha...I swear this is the last question I'll be asking in regards to the subject!:razz:

I'm glad for this post. I've learned a lot. So, the unglazed bottoms act as a release valve, but can also be a port for unwanted moisture? Should we keep them dry down there?

I thought I'd chime in again.

There are so many variables to clays, glazes and manufacturing processes used by a single person, that generalizing for a company or country will often do you wrong.

There are finer quality clays and clazes. Silica is a major component of the bodies of both. All the other minerals mixed in with the silica are sometimes lumped together as "impurities", but this mixing of other minerals is what allows for the durability, colors, melting temperature, etc. to be the parameters of the art and science of ceramics.

Ancient or vintage Chinese porcelain fired at high temperatures with celadon and cobalt glazes are not only artistic feats, but also testaments to the range of ceramics coming out of the same factory as well as the trading networks needed for the raw materials. Refining any material from raw resource extraction to final product is driven by demand, artistry and function. All the materials used for ceramics are stuff that is dug up and sorted and then recombined and used. Lots of labor and travel are involved as the materials migrate from their naturally occurring sources. From one factory may come burnished terracotta with salt glaze and bone china with copper glazes reduced to blushing reds.

Calling low fired pottery as "cheap mugs" only refers to the reduced cost in fuel and time, however they can be far from cheap exhibiting painfully researched glaze testing, sculptural artistry, masterful mold crafting and the pedigree of lineage (aka provenance).

Bars that are giving away barware generally want them to be inexpensive enough to roll into the price of the drink and still turn a profit and not so exclusive or expensive that they get stolen instead. Their needs for them to be around and last like other barware or kitchenware often don't come close the duty cyle of a glass beer pint.

A mug will definitely last longer if the wall is thick, if it is mold casted in all surfaces rather than slip and pour cast, if it is fired higher (fyi they sound different and tend to ring more when struck), if the clay body is quality, if the glazes fit well and finally if they are cared for.

Take those Libby mugs sold at Cost Plus imports and resataurant supply companies. They have simple glazes with understated patterns and are fired consistently with high quality materials. Those are fired much higher and the glazes fit really well compared the Orchids of Hawaii mugs. They'll take lots of wear and tear in industrial dishwashers and will likely outlast most mug companies, however they also are not collected and cared for with the same enthusiasm as a Steve Crane mug so may become rare and loaded with nostalgia because of their ubiquitous presence in many bars.

Regarding keeping them dry: It really depends on the mug. I think a mug should get wet and wear out because it is an active part of living the Tiki dream. Most of the liquids will do nothing as expected, but some glazes/mugs are so low fired and porous that they can stain and even mildew (I had a headhunter mug do that once through the crazing). So, drying them off and taking care of them is definitely a good idea, but not one to be obsessive about. If you have some very valuable mugs, you will have to decide whether you value having them woven into your bar's experiential fabric or if they should be protected and displayed in a case that has a more controlled environment that reduces the swings in temperature and humidity or exposure to direct sunlight.

If you're pretty sure the mug you love is a porous one that is fairly clean & dry, you might consider grout sealer for the unglazed under surfaces that are not exposed to mouth and food.

If you are intent on conserving or preserving rare and collectible mugs, then using glues and sealants can be unwelcome by other collectors who may purchase your collection.

So another dump of clay lore, hope you have fun with your mugs!

Coruscate