Thursday, September 12, 2013

Villainous Vitrification - Turning your Corningware into Obsidian

So, the other day, while I was struggling to clean a particularly stubborn stain from the inside of a 2 quart teapot (P-105), the proverbial "light bulb" went off over my head.

This teapot is pristine on the outside, not a mark, not a scratch.... nothing.

Inside, however, is a different story.  Oh, it's clean....   Especially after I finished with the Barkeepers Friend and then heated up Oxyclean in it and let it soak for 24 hours... But there were still grey "spots" in the bottom.  Thinking they might be hard water stains, I then soaked with CLR for 24 hours.  They were still there.  So, in desperation I decided to try the Pfaltzgraff Cleaner, even though I was pretty sure that they weren't utensil marks... How would you get utensil marks in the bottom of a teapot?  Especially round blotch utensil marks.  Well, the Pfaltzgraff cleaner didn't do anything either, the spots are still there......

So, I took a closer look at the 2 annoying spots in the bottom.

A little closer..... 

THERE, that's close enough.  Can you see it too?  Is it translating well in the photos?  The "grey" looks like it's slightly translucent.  You can still see "white" swirled around within this translucent spot.   My working theory is that these are spots that have vitrified.

OK, maybe I should take a step back for a second and do a little explaining.

When you melt something like, say Sugar, on the stove and it becomes an amber liquid, then you pour said liquid onto a sheet pan and allow it to cool, you have not only formed a hard caramel, you have "vitrified" sugar.  Basically, creating sugar glass.  This happens with rocks as well.  Obsidian, for all intensive purposes, is a vitrified "rock".  ALL rocks, when exposed to intense heat, will vitrify to some degree.

All it really means is that something is melted and cooled in such a way that it remains in an amorphous state instead of re-crystallizing.  Some amorphous mixtures allow more light to pass through them than others.  Such is the case with a mixture of Sodium Carbonate (Soda), Calcium Oxide (Lime), and Silicon Dioxide (Silica Sand); better known as Soda-Lime Glass.

Corningware is based on the DE-vitrification of glass.  Meaning, that it is purposefully crystallized to make "Ceramic-Glass".  This, however, happens after the glass has first been formed and is in an amorphous state. 

The difference is that the glass used to make Corningware has been "seeded", in this case with Titanium Dioxide, so that when the glass piece is heated treated, after the initial forming, the Titanium Dioxide forms millions of little nuclei.  This nucleated glass is heated again and crystals form around the nuclei.  The amount of crystal growth depends on the temperature it is heated to and the duration of the heating.  There you have it, the purposeful de-vitrification of glass.  Pyrex Opal Glass uses some of this same principals but the crystal content is significantly less than that of Corningware, being less than 10%.  Pyrex Tempered Opal Glass (as in mixing bowls and such) are also not formed of Borosilicate, it's a tempered Soda Lime Glass (always has been) and, I believe, it is seeded with Tin Oxide.

In the case of this teapot, I think that it was boiled dry one too many times on the stove, and these areas have UN-crystallize, turning back into glass. Basically, its vitrified.  At least in those spots.  Mind you, this is made of "Borosilicate" low expansion glass to begin with, so I don't think it really harms anything.   It just looks weird on the inside.

As far as why it is ONLY on the inside and not on the outside, I think that has to do with the way that heat moves through and radiates from an object... There were, more than likely, minerals in the water that was being boiled dry that may have had something to do with it as well.

But my theory doesn't extend any further at this point, and I honestly think I pulled a muscle in my brain as it is.   ;-)

Where is your Corningware??


  1. Shane,

    This is Katy with formerly black, now dappled grey, burner shaped marks only inside my Dutch Oven. The marks in your teapot look exactly like what I've now got in the Dutch Oven. Your teapot marks look like the shape of a burner. My Dutch Oven marks also look like the shape of a burner with more of the full circle filled in.

    Over the last month, it took me at least 10 cleaning attempts (various chemicals, heating, and soaking) and hours of scrubbing and my marks did finally, slowly, go from black to dappled grey. I now seem to be stuck at dappled grey. It feels as though the grey discoloration is so bonded into the Corningware that nothing is going to remove it.

    Last night my husband did finally (sort of) convince me that we're not going to be poisoned by grey marks that even the steam cleaner and the Mr. Clean Magic Eraser will not remove. This answer is not me - every other Corningware pan that I've rescued is now pristine. But, I did wash the Dutch Oven twice to remove all chemicals and today I'll put it in the cupboard.

    I did have one discovery that I wanted to share with you. Yesterday's first scrubbing attempt on the Dutch Oven was after an overnight soak with perfume-free Clorox 2 and water. The initial scrubbing may have slightly faded a bit of the dappled grey.

    As an experiment, before I rinsed away the Clorox 2, I added some Sal Suds and Krud Kutters to the Clorox 2. There didn't seem to be any improvement with the dappled grey marks after scrubbing with that mixture.

    I did have an amazing discovery. I had just rescued a piece of Ivy Corningware which had the typical grey/black marks on the bottom. I rubbed a little of the Clorox 2, Sal Suds, and Krud Kutters on the bottom of the Ivy piece and all of the grey/black marks on the bottom immediately wiped away.

    So, In the future, I may start Corningware rehabs with Clorox 2, Sal Suds, and Krud Kutters.

    Thanks for the great blog!


  2. Thanks for the information Katy. I had never thought of using Clorox 2, regular chlorine bleach can dull the shine. I'm gonna have to try and find Sal Suds and Krud Kutters so I can give them a whirl. I have a particularly stubborn 1 1/2 quart Cookmate (SP-1 1/2) that needs a little something extra to get it clean. (It was REALLY bad when I rescued it)

  3. I've officially given up trying to clean the grey splotches from my Dutch Oven. After each experiment, I really scrubbed with my blue scrubber. Nothing happened - the splotches never changed. I think the splotches are definitely bonded into the ceramic of the Corningware.

    I tried boiling mangos (I read that the enzymes in mangos may clean a scorched pan), soaking overnight with dishwasher detergent and water, soaking with borax and water, soaking with Kaboom with Oxyclean, and soaking with Dissolve-It Citrus Cleaner.

    I hate giving up before the Dutch Oven is perfectly restored. I admit that I'm pretty compulsive about all of my pots and pans and Corningware always being pristine.

    Some other things that I haven't tried are boiling/soaking with Coca-Cola and dryer sheets. A few people on the Web mention ammonia and oven cleaner, both of which seem terrible. All of these methods seem pretty useless and I'm very tired of scrubbing.

    Please let me know if you come upon a miracle cleaner.


  4. It amazes me that folks tend to coddle pyroceram cookware.

    I restore my Corninware basket cases using 00-steel wool, green & yellow 3M sponge pads and a sharp hardened steel pick to scrape carbon out of the deeper mold imperfections. Like Shane, I use Barkeepers Friend (but with a green scrubbie) to dissolve away any remaining metallic scuff marks.

    >> Please let me know if you come upon a miracle cleaner

    Magnesium, calcium and iron salts are the likely culprits in the 'vitrified' internal blemishes. The salts diffuse into the surface of the pyroceram at burner temps when the pot runs dry. Try a bit of 600 or 1000 grit wet-and-dry silicon carbide sand paper (use it wet) to buff out the blemishes. If you're skeptical of this consider that Corning manufactured their electrics line by grinding off the bottom of their standard skillets.

    If the blemish is on the outside where you don't want a dull finish then you can finish polish the pyroceram with 3000/5000 Trizact abrasive pads from your local auto parts store.

    1. Thank you for that information, George! This is great!

    2. I use three products - CLR for rust and lime, Cerama Bryte for removing utensil and lid marks and cleaning the bottoms, and sometimes a clorox/water mix soak on the inside to remove any further staining


Note: Only a member of this blog may post a comment.