k77

Hi all, 
A quick update.  I sanded the pots and refired.  The bumps were white and hard in texture.  Somewhat difficult to remove.   I was able to cover them with glaze, but the finished results (where my brush strokes are visible) are not to my liking.  
I have put a fresh load in the kiln today (that I glazed a few days ago). I noticed when putting them in that there were some hard dots on one of the plates.  These wouldn't bother me in a dipping glaze (they would melt and blend in) but I'm wondering if in Stroke and Coat, with its lack of movement, these are the culprit?  Maybe they are bits of dry glaze that I've picked up during a long painting session? Maybe they are small pieces of dry glaze from the bottle (I do try and get every last bit out!) 
So, my working theory for now is that these lumps may actually be from the glaze. This time I've rubbed them out with my finger before firing.  Hopefully I'll see better results after this firing! 
K

Hi all, 
A quick update.  I sanded the pots and refired.  The bumps were white and hard in texture.  Somewhat difficult to remove.   I was able to cover them with glaze, but the finished results (where my brush strokes are visible) are not to my liking.  
I have put a fresh load in the kiln today (that I glazed a few days ago). I noticed when putting them in that there were some hard dots on one of the plates.  These wouldn't bother me in a dipping glaze (they would melt and blend in) but I'm wondering if in Stroke and Coat, with its lack of movement, these are the culprit?  Maybe they are bits of dry glaze that I've picked up during a long painting session? Maybe they are small pieces of dry glaze from the bottle (I do try and get every last bit out!) 
So, my working theory for now is that these lumps may actually be from the glaze. This time I've rubbed them out with my finger before firing.  Hopefully I'll see better results after this firing! 
K

Hi all, 
A quick update.  I sanded the pots and refired.  The bumps were white and hard in texture.  Somewhat difficult to remove.   I was able to cover them with glaze, but the finished results (where my brush strokes are visible) are not to my liking.  
I have put a fresh load in the kiln today (that I glazed a few days ago). I noticed when putting them in that there were some hard dots on one of the plates.  These wouldn't bother me in a dipping glaze (they would melt and blend in) but I'm wondering if in Stroke and Coat, with its lack of movement, these are the culprit?  Maybe they are bits of dry glaze that I've picked up during a long painting session? Maybe they are small pieces of dry glaze from the bottle (I do try and get every last bit out!) 
So, my working theory for now is that these lumps may actually be from the glaze. This time I've rubbed them out with my finger before firing.  Hopefully I'll see better results after this firing! 
K

They seem more like bumps to me.  As if there are grains of sand under the surface of the glaze in places.  There are however also one or two pinholes, which is something I've been battling for a while.  These white bumps seem different from the problem I usually have! 

Hi Callie, 
Thanks so much for taking the time to help me!
I will check the kiln shelves, but I would be surprised if this is the issue, it hasn't been an issue before......
I didn't trim the surface of the plates as such (I hand build), but...... I did use a metal rib to smooth out the clay.  Now I think about it, this did expose the grog on some occasions (which I did smooth out with a rubber rib - but maybe not enough?)  Perhaps I should only use a rubber rib for this clay?? I usually use clay without grog so this is something I perhaps haven't been paying enough attention to. 
I'm certain this clay is the one with grog.  It is part of the W11 series.  W11 is without any grog.  Then W11 25 02 has 25% grog at a a size of 0.2. 
Seems I need to pay attention to how I'm making the plates so as not to expose the grog.  But do you have any other thoughts on the firing temperature.  I've been surprised to realise that the suppliers list such inconsistent results. 
Thanks again
K
 

Hi Hulk, 
Thanks so much for taking the time to answer (and in so much detail!)   
Although the clay is not poking  all the way through the glaze, my sense is that the bumps are coming from the clay - rather than from the glaze.  In which case, off gasing seems likely.  I will try and change the firing schedule somewhat and add a hold in the bisque as you suggested.
The possibility of it being too hot makes sense to me, it crossed my mind when I saw the results.  However.....
Having read your reply, I am now somewhat confused about the correct technical data for this clay.  In fact I bought the clay from Cigale and Fourmi that lists the water absorption as 1.3% @ 1300. https://www.cigaleetfourmi.fr/terres-a-gres/3996-gres-blanc-w11-chamotte-25-0-02-mm-sac-de-10-kg.html
Confusingly, when clicking on the link to the technical data on that page, the information is different.   (2.7% @1300).  
Another of my suppliers has the same product but lists the water absorption of the clay as 1.2% at 1260.  https://www.ceram-decor.fr/en/ARGILES/8989-GRES-BLANC-CHAMOTTE-25--002---W2502-.html  I'm certain this is the data I relied upon when choosing the firing temperature.   Knowing that my kiln fires hot, I chose 1260 in order to have proper vitrification and achieve porosity of around 1%.   The technical details on that page list the water absorption as:
1000°C : 13.1% 1100°C : 7,2% 1200°C : 2,4% 1300°C : 0,7% I can't find the technical details for this clay on the Witgert link you sent.  It seems I need to resolve this, as possibly a lower firing temperature might be the answer (subject to achieving vitrification).
I'd welcome any other guidance or suggestions.  
Many thanks!
Katherine