Waxing and dishwashers

[macca]

Hello,

If I add wax to the bottom of bisque to prevent glaze sticking to kiln shelf rather than using a stilt, will this not leave the base porous therefore not dishwasher proof? Thinking for mugs and plates that have thin rim to sit on. Apologies in new to ceramic craft!

[Hulk]

Hi Macca, good question!

What clay are you using, and how fired? How much area are you waxing? If using grogged and/or sanded clay, are you burnishing the exposed parts?

Just about all the ceramic in our kitchen (some is handmade stoneware, the rest is commercial stuff) has an unglazed foot, be it a thin ring or more - like the entire foot. Generally, I'm cutting a ring and lightly glazing inside the ring, which leaves just the bottom and outside of the ring itself unglazed. I do burnish that ring after trimming - smoother and tighter, eh? Given that the published absorption of the stoneware we're using ranges from ~2-6%, an' nothing's blown up yet, I'm thinking it's good. I'm curious what other forum members have to say...

 

[JohnnyK]

Welcome to the Forums, Macca.  First of all, what cone are you glaze firing to? Are you glazing the bottoms of your pieces and then waxing or are you waxing to keep the glaze off the bottoms? Are you using a runny glaze or one that will stay in place? In any case, as far as the wax is concerned, it will burn off before it could cause any kind of problem with the clay vitrifying.

JohnnyK

[Bill Kielb]

Generally folks wax the bottoms of their wares so they can glaze and wipe off any  excess glaze  from the waxed area thereby preventing their wares from sticking to the shelves. Almost all wax burns away  between 400-800 degrees F so the wax is long gone before vitrification of the clay. Best insurance against water absorption is full vitrification. 6% would be way too much in my view, even 3% not very good.

of course if you are using low fire clay it does not fully vitrify and the completeness of your glaze becomes all that much more important.

[macca]

I'm glazing bisque bought commercially which has been painted by people in my wife's mobile craft business.  The Glaze is clear and quite thick so having no running issues. Firing to cone 6 with 20 min hold. So far have used Stilts and had to sand the sharp glaze points off the bottom however for plates this week not work as I need to stack them.  I just plan to wax the ring they sit on to prevent glaze sticking to the kiln racks but will they still be dishwasher proof? The Glaze itself is very robust and dishwasher proof. Like Hulk states I have noted commercial plates have the ring at the bottom but wondered in the ring was coated with anything else after glazing? 

[Hulk]

I don't know if any treatment for unglazed fired clay to limit/prevent absorption; other forum member(s) might know?

As we don't (yet) know what clay is being used, perhaps a simple test? Try weighing out dry finished piece (my digi scale reads to nearest .01 gram); soak in water for a reasonable washing time, perhaps 20 minutes; reweigh. How do the pieces compare to commercial selection of pieces?

Hmm... might try that out on my own pieces - I leave a substantial ring unglazed.

[Bill Kielb]

I am not sure I understand  so first

cone 6 plus 20 minutes is about cone 7 which might over fire cone 6 glazes and make them move a bit.  I think that might be a typographic error

if I understand correctly you have been firing on stilts with pins  (Roselly stilts) so that the entire piece has been completely glazed and now for the plates you wish to wax the foot, dip th plate in glaze then wipe off any excess glaze from the foot. This is quite common and results in a glazed plate everywhere but the foot.

the wax keeps the glaze off the foot. The wax serves no function other than to keep the glaze off the foot. It will burn away early in the firing and will not protect anything. Just so we are clear about why wax is used.

as to being water resistant this foot area is commonly not glazed and if the clay is fully vitrified and minimally absorbent after vitrification it usually poses no problem.

commercial plates have the ring and they are generally Not glazed or coated with anything.

...and I would add Hulks absorbency test is certainly a step in the right direction to figure out what the clay will do..

last note, I am not sure I understand the stack them comment so stacking glazed plates and firing them will  likely result in one solid stacked plate. Careful!

[macca]

Thanks for the information chaps you have been a great help, I will try the absorbency test. I am using Plate stacker so they won't be contacting each other. I am using a digital cone and holding at 990 degrees c as per the glaze instructions and it seems to be working well. I'll test the weights after my next fire and do the absorbency test. I'll repost with the results.

Thanks again

Macca

[Chilly]

1 hour ago, Bill Kielb said:

last note, I am not sure I understand the stack them comment so stacking glazed plates and firing them will  likely result in one solid stacked plate. Careful!

@Bill Kielb,  @macca did say "kiln racks", so presumably they will not result in one solid piece !!!

 

And I agree with the absorbency test.  If @Macca searches these forums, I'm sure the subject has come up before.

[Chilly]

Did you mean cone 6 or cone 06?  Big difference.

[Bill Kielb]

3 minutes ago, Chilly said:

Did you mean cone 6 or cone 06?  Big difference.

990 C is cone 06, great!

[Min]

1 hour ago, macca said:

I am using a digital cone and holding at 990 degrees c

A digital cone? hmmm, I would really suggest adding some cone packs to verify your thermocouples readout.

Just about every lowfire clay will still be very porous, except for fritware. What some potters do who work in lowfire is to use terra sigillata on unglazed areas. It doesn't totally seal the clay but it cuts down the water absorption a bit more than just leaving raw clay.

For your absorption test you can start with doing a simple one, place a few drops of ink or food colouring on an unglazed area then leave it sit for an hour or so. Rinse it off and see how stained the clay is. It will give you a quick idea of porosity. (try it with a vitrified clay, if you can, to compare the two) For more accurate testing take an unglazed test tile, strip of clay or whatever and when it's straight from the kiln weigh it really accurately. Then put it in a pot of water and boil it for an hour, turn the stove off and let the test piece cool down in the pot. Remove it, pat it dry and weigh it again. Absorption = (saturated weight - dry weight) divided by dry weight X 100   You want the absorption to be below about 2.5% at the highest, really don't think your lowfire clay will be though, more like in the 10+ range.

[neilestrick]

Yes, big difference between cone 6 and 06. At cone 06, your clay will not be vitrified, so any unglazed area will remain quite porous. That's not a good thing. I would continue to use stilts. Commercial dinnerware has a small unglazed area on the foot ring, but that exposed clay is vitrified (water tight) so it's not an issue.

[Babs]

Not sure how durable but a potter used to seal to g with slate sealer or the likes after firing.......but that was on vases which wouldn't be in d.washer all the time... just saying

[macca]

Apologies it is a 06 cone I'm using in a digital kiln. The bisque I'm using is purchaced from bisque direct as white unglazed earthernware that has been produced commercially. They advise glaze between 950c to 1050c.  Is the clay vitrified when it is glazed or when it is first fired? 

I will do an absorbance test on an unglazed tile then I'll have my answer. I'm using kiln racks for the plates so they arn't contacting each other. 

[Chilly]

On 12/8/2018 at 8:40 PM, macca said:

Apologies it is a 06 cone I'm using in a digital kiln. The bisque I'm using is purchaced from bisque direct as white unglazed earthernware that has been produced commercially. They advise glaze between 950c to 1050c.  Is the clay vitrified when it is glazed or when it is first fired? 

I will do an absorbance test on an unglazed tile then I'll have my answer. I'm using kiln racks for the plates so they arn't contacting each other. 

Earthenware is usually un-vitrified.  It will stay damp and leave a ring if you put a freshly washed and dried mug on a wooden table.  That's why it is glazed all over, with just 3 pinpoints from stilts.  Stoneware is not (normally) glazed on the foot and should be vitrified if fired to the correct cone.  Most cheap tableware (in the UK at least) is earthenware.

Back to the original question of "dishwasher-proof" - I have some earthenware mugs that go in the DW and are OK.

[Bill Kielb]

Just to add - the misnomer of dishwasher stuff. Interestingly enough if a glaze has a questionable chemical composition the dishwasher is generally its worst enemy. The mythical lemon test is more often than not mythical, as soap and water washes things away for a reason. Especially non durable glazes. So all the more reason to make sure your Ware is vitrified and as non absorbent as practical. Also a good reason to have a staple of known durable glazes at the ready.

 

[Mark C.]

Low fire bodies are notorious for absorbing water. Low vitrification rates and high absorption rates are the norm in these Earthenwares.Most 06 ware have glaze all over them to try and aid in all the above issues. I'll leave this to others to sort out as I left low fire about 45 years ago as just an art clay medium as the functional wares are just to fragile for me.