Piholes On Slipcasted Greenware, Pinholes On A Glaze

[natanata]

Dear everyone,

I am quite new to a slip casting technique. Have made several plaster molds for casting porcelain.
And had some success, but recently I have noticed that some of the greenware gets tiny pinholes 
and then, (because some cups doesnt have it) there are SOMETIMES also pinholes on the glaze.

If i got it right, those tiny pinholes are the result of air bubbles or pieces of dust in the casting slip, right?
But i wonder, do those pinholes influence the glaze? I am a bit confused,

because some of the porcelain cups are not having those pinholes on the glaze and some do have.

The thing is that the kiln in the studio where i used to fire my work is very old, and as a kiln technician said,

it fires hire than it should and moreover fires unevenly.

I wonder if this could be the reason for the pinholes on the glaze surface? Or maybe pinholes on the greenware? Or both?

Do you have any ideas? Or similar experience?

Thank you in advance!!



 

[High Bridge Pottery]

I think your right on the bubbles. It looks like the glaze has melted into the holes, if they are the same holes that were there before the glaze.

 

When you are mixing the slip are you trapping bubbles? 

[GiselleNo5]

I've had some issues with pinholes when slipcasting. I've even had pinholes that were invisible until the glaze fire when they suddenly sucked glaze in.

 

I've also had badly pinholed bisque that fired fine ... I think sometimes the glaze is forgiving and sometimes it isnt.

 

My main problem with casting slip was the porcelain slip. I've made many items with stoneware slips and had no pinholes. So it might be worth trying out a different kind of casting slip.

[Mark C.]

Tips to avoid slip ware pinholes-use dust free dry molds-pour slip in slowly avoiding catching air on the fill.

After casting while greem inspect for pinhols and sponge thme out or fill them and sponge them.

When glazing make sure wares are dust free. If you can see pinholes after glazing when glaze is dry rub them out before firing.

Pinholes are easier to deal with as soon as they are spotted in the process.

I had a porcelain casting business long ago and we at one time cast many thousands of a 5 inch aromatherapy lamp in cone 10 porcealain.

Mark

[natanata]

I think your right on the bubbles. It looks like the glaze has melted into the holes, if they are the same holes that were there before the glaze.

 

When you are mixing the slip are you trapping bubbles? 

when mixing the slip i wait till there are no air bubbles, 

i wonder if i should maybe sieve the slip every time i cast another ware, as i am always putting the rest of the slip into the main batch.

 

[natanata]

Thank you, Marc, Giselle and Hight Bridge Pottery. 
using all the advices i will see if i can change something next time!
n

[Mark C.]

I have never sieved slip ever. The key is pouring it so bubbles do not get trapped working it. I poured with a gas pump nozzle on my slip pump but and tip the mold slightly -when pouring smaller single molds I use a spouted container and tip the mold so it fills evenly with no bubbles.You can also tap the mold so bubble rise to top.Never had many issues with this-is your slip adjusted right with a slip hydrometer and viscosity cup before pouring????

Mark

[natanata]

I have never sieved slip ever. The key is pouring it so bubbles do not get trapped working it. I poured with a gas pump nozzle on my slip pump but and tip the mold slightly -when pouring smaller single molds I use a spouted container and tip the mold so it fills evenly with no bubbles.You can also tap the mold so bubble rise to top.Never had many issues with this-is your slip adjusted right with a slip hydrometer and viscosity cup before pouring????

Mark

Mark,

thanx, yesterday tried casting again using all the advices and it turned out well!

actually, no, i am always mixing the porcelain powder with water as in the instruction - +45-45% water to the porcelain powder.

but now it looks like it works anyway. 

 

Just another question, when you mix a new slip, how long should it sit before you pour? 

thanks a lot!!

nata

[Jeff Longtin]

Nata,

 

Pinholes in casting slip are the result of too thick a slip. 

 

Its a delicate process but you can thin a slip two ways: adding more water or adding more sodium silicate. If you add too much water you may see more cracking in the mold. If you add too much sodium silicate you may see the slip gel up too much.

 

Thinning the slip with water is usually the best "first step". 

 

Another trick is to moisten the mold surface with water before casting. This was a trick given to me by a doll maker many years ago.

 

good luck

 

Jeff Longtin

[Magnet]

This is an old thread, but I wanted to correct some potential problems that could arise from advice here:

Slip problems arise from one or both issues:  weight (specific gravity) and thickness (viscosity). Any slip recipe has an "ideal" weight and thickness. Both of these properties can fluctuate as slip ages.

If your slip is too heavy, you can lighten it by adding water. BUT: Do not add water to your slip without checking specific gravity by weighing your slip -- not a hygrometer. Research has proven this, and it's what you'd be taught if you went to school to be a ceramic engineer. Your slip recipe will have an "ideal" specific gravity, likely between 1.75 -1.78

If your slip is too thick, you can fix it by adding deflocculant. But: Do not add deflocculant without checking viscosity with a viscometer (unless you're adept at estimating viscosity by look/feel). 

Bubbles in slip and castings result from air being suspended in the fluid. This often happens with a new batch of slip, when the dry materials have not fully soaked, and if the slip remains too viscous (too thick) to allow the bubbles to easily gather and rise. Proper mixing and proper deflocculation typically remedies the bubble/pinhole issue. Check the specific gravity and viscosity of your slip before you begin pouring for the day, and you'll be able to fix these issues before they arise. 

Then you can rock the casbah. 

[AscotLady]

Thank you Magnet,  1.75 - 1.78  that must be USA measurement-

I know this is an old thread..... but....   Ive just started slipcasting, what fun.  Beginners luck and all the vases I cast were perfect, bought tubs of porcelain slip.  Then tried stoneware, also purchased, too heavy, too everything, vase OKish.    Now tried to go back to basics and make it all myself.   Instead of persisting with own dried clay, I purchased powdered slip casting stoneware, found recipe on the net, in all my (many)reference books, came up with what sounded right,  diluted the weighed gram  soda ash, checked the pint weight, weighed (ahah!) the sodium silicate.  Left for 24 hours after passing it over a magnet (?why),  added half the amount of dispex just because it sounded too much.   Voila, nice pouring slip, set up in twelve minutes, poured out smoothly, oh-some odd pinholes inside the vase ( havent turned it out yet ).     So I look up pinholes and come to this thread.  

Now more reading and a lightbulb moment, USA oz, and the number of oz in a pint are very different to UK measurements, still called pints and oz.    I also weighed the sodium silicate, but used a ml syringe for the dispex (Darven in USA).   I used the UK oz from a USA recipe, does that matter anyway.......  The problem now is the pint weight for the clay noted in the brochure is 35-35.5 oz - of course this is UK oz.   My clay weighed just over the 35.5 I thought I had performed a miracle,   1Kg = 1 pint in UK =35.5 oz.     How does this translate into USA measurements for Pint weight- I think I may have mixed up the countries/recipes.      As you can tell maths has never been my strongest suit.    So now my clay feels a little heavy, do I top up with Dispex or add a little water? Also pint weight now a little low at 33.1 oz (UK), do I add water or Dispex?

No wonder I have been having trouble baking cakes, using USA online directions, ‘old money’ mixed with metric +- difference in USA and UK pints and oz.  Bit like the Spacestation mix ups with measurements maybe?

[liambesaw]

1.75 - 1.78 is a measurement of specific gravity, its usually done in metric.  It's a comparison between the weight of water and weight of your medium (slip in this case).  You can Google how to measure the specific gravity.  You would determine whether or not you're needing to add more water or add more darvan to thin it.  If the SG is over 1.75, add water to thin the viscosity.  If it is 1.75 and still too thick or thin, add your deflocc or flocc.

[AscotLady]

Thank you for your quick response.  I will check this out before casting again and let you know the result.

[AscotLady]

So,  it appears tgat SG is the same as ‘Pint weight’, it doesnt matter what you weigh it on as long as its accurate and using the same scale for the two weights.  The containter doesnt matter either just use the same mark to check the weight.   In the UK 1000ml is 1Kg and is a good weight for measuring slip, if you use too small amount it is less accurate.    You ‘tare’ the container on the scale first, then add the slip, so no complicated calculations.   The same applies for SG for glaze ( you need to know the right SG for the glaze you are using), if you have made it yourself you can test it when new and check each time you use it to adjust.  Likewise with slipcasting, test the SG each time.   I will need to judge the viscosity by the ‘double or single cream’ feel, and adjust with tiny amounts of dispex up the maximum for the clay I am using.

So in UK. Clay should SG at 35-35.5 pint weight

In USE Clay with 100ml 1.75-1.78

Glaze 1.4-.1.6.   Rec’d for USA, but this is dependent on make.  I need to check my glazes spec and see if it is available for glazes here, or just check the ‘new pot’ or my first batch made.

Thank you for all this great information on this site, I love it.