What is “Vitrification”, and Why Should I Care?

By Janet (imapigeon) on June 14, 2010

We container gardeners are always on the prowl for the perfect ceramic pot. We want it to last a long time, and provide a healthy environment for each plant. Of course, one of the most critical elements for container plants is soil moisture retention. You already know that plain terra-cotta pots have to be watered frequently, but that pots with a shiny glazed surface can go longer between waterings. Ever wonder why? It's all determined by the pot's clay chemistry. High-fired stoneware and porcelain clays contain large amounts of silica. When these clay bodies become vitrified (or “mature”), they are no longer porous.

Vitrify:   “To convert into glass or a glassy substance by heat and fusion” (Webster).

Terra-cotta / earthenware clays and casting slips are not formulated to vitrify during firing, and must be glazed on all surfaces to protect the pot from moisture.  But glazes on these low-fired clays only coat the surface.  The smallest crack or pinhole in the glaze will allow dampness to seep into the clay, and over time the pot will be damaged.  Freezing and thawing cycles will have a significant effect on low-fired pots, so they should be protected during the winter where temperatures dip below freezing.

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Vitrified pots are much less subject to freezing temperatures, because, like glass, they don’t absorb any moisture.  The expansion of standing water as it freezes is very likely to crack even high-fired pots, so it’s important to always drain fountains and water gardens in regions where winter temperatures reach freezing.  But with good drainage, vitrified pots should be able to withstand freeze and thaw cycles. 

When properly fired to maturity, the glaze on a stoneware or porcelain pot actually bonds chemically and physically with the clay, rather than just coating its surface.  This makes the glaze extremely durable.  Vitrified pots don’t need to be glazed on the inside.  Personally, I think plants are happier in unglazed pots, because they have more texture for the roots to grab onto.

So how can you tell whether a pot is vitrified or not?  Using your knuckle, knock on the pot wall in several places.  If it’s vitrified, it will make a distinct ringing sound that lasts for a second or so.  If it’s not vitrified it may still ring, but the sound won’t last as long---or it may sound like “tunk”, which means the clay is not mature and the pot will not hold up well over time.  The knuckle technique can also help you identify a cracked pot----the flaw will affect the ringing sound.  With a little practice, you can learn to distinguish these differences in sounds, and you’ll soon be able to choose the best pot in the whole stack!

Typically, earthenware pots are much less expensive than those made of stoneware and porcelain.  But a good pot can last a lifetime---so practice the knuckle technique.  Next time you buy a vitrified pot, you can feel confident that you’re making a worthwhile investment.

(A big "Thank you!" to Sharran and zenpotter for their comments on the draft of this article!)

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Related articles:
ceramic pots, clay, container garden, containers, earthenware, planters, porcelain, pots, stoneware, terra cotta, terra-cotta, vitrification, vitrify

About Janet
I've been involved with clay off and on since 1972----I love getting my hands in the mud! I've taken classes at several colleges, managed ceramic shops, and taught many students. After retiring in 2009 from environmental, health and safety, I began dedicating several hours a week to clay projects in my little studio. I especially like throwing planters, bowls and lidded jars, and I've made hundreds of handpainted tiles for my house. I do much of my firing at home, where I have both electric and gas kilns.

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Comments and discussion:
Subject Thread Starter Last Reply Replies
Clay Pot clintbrown Sep 27, 2010 6:58 PM 3
Excellent article, Janet Sharon Jun 14, 2010 8:23 AM 1
Thanks for this great advice UniQueTreasures Jun 14, 2010 8:22 AM 4
yes! Boopaints Jun 13, 2010 9:02 PM 5

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