Porcelain - Why does only the bottom of my work have an eggshell shine?..

[Chiv]

Hi all,

I have been using a porcelain casting slip - ready made. I have made 'test tiles' and have found that the bottom of them have an eggshell shine to them, compared to the tops that have either been left unglazed and look and feel like the usual matt quality, or have had a transparent glaze added.

- Is this to do with using alumina hydrate directly on my kiln shelves to avoid them sticking - does this contribute to this...? Or is this irrelevant?

Appreciate any advice - new to porcelain and working out what works / the whys/ dos and don't.

As a side note - I like the eggshell shine, curious how to achieve this...

Thank you!

[Min]

Could you post a picture? Sounds like the underside has had more heat work that the top of the tiles. There seems to be quite a range in what people call porcelain, is this a white glassy, slumpy, vitrified porcelain? 

Alumina hydrate won't have contributed to this.

Welcome to the forum.

[Chiv]

hey Min,

thanks for getting back! and thanks for the welcome

It was a ready made casting slip bought direct from a clay seller, just says Porcelain Casting Slip 1260c max... I haven't any more info at the mo. And I am yet to fire full scale pieces to see final result. However the alternate sides of all are definitely only matt. I'll try and find a more comprehensive label for it. 

Excuse bad quality photo, tried adding another - but can't export it small enough right now to be accepted.

You'll see alumina has got stuck in to it also. I added it directly to shelves without water, maybe I added too much..

[Mark C.]

I think you need to slip cast a form and then see how it looks. Since its commercial mix yiu will be stuck with however it comes out anyway .

[Min]

It's hard to make out in the picture but is the alumina gray? Are you 100% sure it was alumina and there wasn't a labelling mixup or something?

[Chiv]

Hey Min and Mark - excuse delay, my notifications weren't set correctly to see replies, hopefully fixed this now.

Yes the alumina is white, the lighting is deceptive there. 

I'll go ahead with slip casting and see.

Any advice on how much alumina hydrate I should be using on the kiln shelves? I wonder if I use too much... I'm not firing huge amounts, so using alumina directly on shelves appealed to me aka can pour back in to tub to use again, and no worry of a very highly used kiln and it going everywhere on different pieces. 
At the moment I'm putting a quite substantial layering over the shelf, any advice on the 'thickness' that's realistic/necessary? eg. in cm, or how it should look top down?..(I was conscious to not let any pieces stick so maybe I'm going a little overkill..)

Thanks again inadvance!

[Chilly]

3 hours ago, Chiv said:

alumina hydrate I should be using on the kiln shelves? I wonder if I use too much... I'm not firing huge amounts, so using alumina directly on shelves appealed to me aka can pour back in to tub to use again

Take care working with dry powder.  Wear a proper respirator.

 

[Mark C.]

You just need it to be thin tghin coating of powder-if it gets into the glaze it will not melt so be super carefull-kiln wash is easier to use than powder every time.

[Chiv]

Hey Chilly - yes I make sure to do so

Mark - thanks for this. Do you make your own kiln wash or buy it? Could you recommend how/where at all? I find it a bit of a minefield. I see some solid recommendations and then others say how it flakes and ruins things etc.

[Min]

@Chiv, there is thread here talking about kiln wash recipes. Kiln wash discussion starts a few posts down in that thread.

On 2/28/2021 at 5:59 AM, Chiv said:

Any advice on how much alumina hydrate I should be using on the kiln shelves? I wonder if I use too much... I'm not firing huge amounts, so using alumina directly on shelves appealed to me aka can pour back in to tub to use again, and no worry of a very highly used kiln and it going everywhere on different pieces. 
At the moment I'm putting a quite substantial layering over the shelf, any advice on the 'thickness' that's realistic/necessary? eg. in cm, or how it should look top down?..(I was conscious to not let any pieces stick so maybe I'm going a little overkill..)

If you want to use alumina hydrate as an added layer of protection in addition to using kiln wash (also known as batt wash or shelf primer in the UK) you can mix up some cold wax resist with alumina hydrate and brush this on the bottom of your pieces. Approximately 1 1/2 - 2  teaspoons of alumina hydrate to roughly a cup of wax resist, add some food colouring to it so you don't mistake it for regular wax resist and don't get any on your glaze or you'll make a very rough sandpaper like texture. The alumina hydrate will settle out of the wax resist fairly quickly so keep it stirred up while using it.

[Chiv]

@Min Thank you for this, yes I had heard of brushing beneath - although didn't know of the colouring tip. I'm checking out the wash recipes now - thanks!