Vitrification

[Ret]

Please forgive the noob question and also if this has been answered before. . If you wanted to make doubly sure that functional wear was waterproof and food safe would firing a clay to its vitrification cone after a bisque fire and then firing it again with food safe glaze on it add an additional layer of protection? 

 

Example: cone 7-10 clay. 

Bisque fire?

Fire to cone 7

Glaze then firebto cone 6-5 for glazing .

 

Thank you!!

 

 

[Mark C.]

No noob qestions really

No you bisque fire cone 04-06 for example

then clay is absorbent so it can take glaze (absorbs and water dries leaving glaze on wares) 

then fire to vitrifaction cone say cone 5-6

ops I noticed you said cone 7-10 clay so this clay need to glaze fire to cone 7 or 10 to be vitrified -That is a strange range for a clay-7-10 Testing how absorbent it is will be needed  to se how vitrified it is say at cone 7-this sounds like cone 10 caly to me

if you fire clay to cone 6 before glazing you cannot get the glaze to stick

In terms of vitrification you only fire once to that cone  with the glaze on it-more fires will not help it -but in fact weaken it and make the glazes way different.

I'm not a big You tuber but I think thios must be covered in somebody's video -or a good clay book will cobver all this as well

In ceramics there are 10,000 ways to screw it up so start with the basics that are all followed by most of us. After they work then try the next thing . Testing limits is always a good idea but not this one.

For me I fire cone 08 bisque porcealin (eyeball it thes e days) except in a electric kiln. Then fire to a soft cone 11 in reduction always with cones.

[Hulk]

Hi Ret!

Each clay will/should have a cone* where it will be as vitrified as it will get without losing color, expanding, bloating, warping, blistering, or otherwise bein' overfired.

That ideal amount of heat work - where as vitrified as possible without being overfired - would typically be more specific than a four cone range; more like a half cone or so. Finding the target cone might take some testing.

How waterproof a clay is does vary; some stonewares are rated above 5%! Low absorption clay, less than 1.5%, is available.

Bisque fired ware should be easier to glaze than fully fired - much easier.
I don't see any advantage in firing to glaze target twice.

A well fitted, durable, non-toxic glaze, on fired to vitrification clay, that's a good objective.

Vitrification (digitalfire.com)

*the "cone" reflects "heat work" achieved - how much time at temperature; it's both, time and temp.
Do it once.

 

[Ret]

The reason I ask this is because I have a clay body, Highwater phoenix clay to be precise, that is coneb7-10. I would use cone 6-5 glazes on it, bit thought firing to cone 7 would vitrify it sufficiently to add an extra layer of food and especial microwave safety  I worry about that A LOT.

 

Thanks for all of the suggestions!

[Mark C.]

After looking at this clay from High-water  (web site) -its cone 10 clay to get to full vitrification 

not cone 7 -if you want to go to cone 7 you will have to test the vitrification via a bar test(boiling and weighing)

Its also an open raku clay and sculpture clay which means either lots of grog or sand-nither makes for tight clays.

looks like they are out of it as well. 

[PeterH]

 https://highwaterclays.com/products/wet-clay-phoenix-c-7-10

[Bill Kielb]

9 hours ago, Ret said:

The reason I ask this is because I have a clay body, Highwater phoenix clay to be precise, that is coneb7-10

I think what Peter said above sums it up. For that clay fire to cone 10 with a cone 10 glaze to be as waterproof as possible. This will ensure it is fully fired so as strong and dense as it can be which translates to less issues with absorption, weeping, microwave safety, dishwasher safe, etc…. Use a durable food safe glaze (no cadmium, lead, other toxic oxides) that fits the claybody (no glaze defects, crackle, crazing, etc…) and it will be as good as it can be for food products and such.

Fire to a temperature less than fully vitrified and the clay will be less strong, more absorbent. Lots of folks shoot for less than 1% absorption.

[JohnnyK]

How much of this clay do you have to work with? Is it one bag...5 bags... 10 bags? If it's just a small amount, why not just switch to a ^5 or ^6 clay and not worry about it?

[Mark C.]

6 hours ago, PeterH said:

 https://highwaterclays.com/products/wet-clay-phoenix-c-7-10

Thanks for posting what I saw on that site-its really a cone 10 clay as I said

[Min]

Absorption figures from many clay suppliers have a fudge factor of around =/- 1% so for claybodies being used for functional work its a good idea to run your own absorption tests under your own firing conditions. 

Link here on how to do an absorption test by weight if anyone needs it.

[neilestrick]

Clay bodies generally need to be fired to their max rating in order to be sufficiently vitrified. If you plan to glaze fire at cone 6, then you need a clay body that maxes out at cone 6, not a cone 6-10 body, which will be under-fired at cone 6. As for the clay you have, at cone 7 it will be under-fired according to their chart.  You want to get the absorption rate under 2%, ideally under 1.5%, if you plan to make functional work.

[Ret]

On 12/21/2021 at 12:44 PM, Min said:

Absorption figures from many clay suppliers have a fudge factor of around =/- 1% so for claybodies being used for functional work its a good idea to run your own absorption tests under your own firing conditions. 

Link here on how to do an absorption test by weight if anyone needs it.

Hugely hugely helpful!!!!

[Ret]

Thank you so much for educating me!! I only have one bag of this and it did occur to me later that a higher cone of glaze might do it, but my kiln maxes out at cone 8 so suggestions on switching clay bodies were spot on! Thank you all so much for the help!

[baby-egg]

On 12/21/2021 at 1:01 PM, neilestrick said:

Clay bodies generally need to be fired to their max rating in order to be sufficiently vitrified. If you plan to glaze fire at cone 6, then you need a clay body that maxes out at cone 6, not a cone 6-10 body, which will be under-fired at cone 6. As for the clay you have, at cone 7 it will be under-fired according to their chart.  You want to get the absorption rate under 2%, ideally under 1.5%, if you plan to make functional work.

So I am wondering what are higher absorption rate clays mostly used for? Sculptures?

I make functional work, and when I started picking different clay bodies to experiment with, I wasn't paying attention to the absorption rate, only to find out that my mugs were letting water seep out! I now always check the absorption rate. 

[LinR]

Use your cone 10 clay for items that don't have to hold water - candle holders, tea lights, garlic pots, spoon rests, key or ring caddies, animals, dragons, mushrooms ,little houses, pendants, masks.  The choice is endless.  Glaze those at cone 6.  Then buy yourself proper cone 6 clay for any functional work.  Lin

[oldlady]

highwater makes little loafers which is a good, white, smooth cone 6 clay.   i have used it for years for both throwing and handbuilding.  with the right glazes, thank you Min!, it is very practical for functional work.  look at my albums to see how flexible it is.