High Bridge Pottery
-
Posts
2,619 -
Joined
-
Last visited
Content Type
Profiles
Events
Forums
Blogs
Gallery
Posts posted by High Bridge Pottery
-
-
Never had a problem cleaning a sieve but I have used a brush for white glaze and found it still had colour hidden in the middle. You can send glaze past an electromagnet if you want to get rid of all the ferrous bits.
-
On 11/15/2024 at 1:59 PM, Amy1991 said:
There's a film on the top of some tanks and not on others but it's not crystalised its more like a dark thin cracked skin. Then in another tank it looks as if its clumped together in a big block in the middle where the blades are.
You would probably have to sieve it to find any crystals. I thought it was flocculating over time at first but a skin forming does sound like possibly too much sodium silicate or you need to add a little bit more water to those batches and use a slightly lower pint weight. Are you around 36oz/pint?
I wonder if the big clump forming is just the same film/skin getting caught and clumped around the blades.
Sodium/Ammonium dispex could be another option to try and see if the issue is with sodium silicate.
-
I would think any commercial liquid gum solution would contain some additives to stop it degrading/molding already.
-
If high speed mixing doesn't get the viscosity back to normal then it could be down to varying particle size distribution (I forget if you should add more or less water, I think more water) or it could be down to calcium ions going into solution. If you take a sample off the thick slip and add more sodium silicate does any calcium silicate crystalise out? Might give a hint to if it's calcium based flocculation or particle size distribution related.
-
On 11/12/2024 at 6:11 PM, Min said:
Can't say I've ever come across anything to indicate bentonite would deflocculate a glaze.
I guess my concept is adding bentonite could introduce more repulsion into the glaze because of low isoelectric point and make packing efficiency better from tiny particles causing the glaze to be more fluid. Maybe my understanding is wrong but it sounds logical to me
-
-
A good book if anybody wants to go down the rabbit hole of rheology - https://books.google.co.uk/books/about/Rheology_for_Ceramists.html?id=exH9AgAAQBAJ&redir_esc=y
It's not exactly the right way to look at bentonite and epsom salts in the same way. I think fine particles in bentonite help stabilise suspensions due to increasing the particle size distribution and packing efficiency whereas epsom salts (Mg2+) locally neutralises negative surface charges allowing van der waals forces to create floccs.
I could see a situation where 2% bentonite would increase how long things stay in suspension while also deflocculating the glaze.
A bit from the book
QuoteWhen particles are suspended in fluids, especially polar fluids like water, their surfaces can be expected to exhibit electrostatic charges. Deflocculating chemical additives can be used to enhance net charge density to higher positive or higher negative values. Flocculating chemical additives can be used to cancel electrostatic surface charges.
The mineralogical types of particles, suspension pH values, and the concentration and type of additives all affect the sign and charge density of electrostatic surface charge. Each different mineralogical type will have an isoelectric point (IEP). The IEP is the pH at which the net surface charge on a suspended powder (slurry) is zero.
-
Quote
with formation of rounded shaped zircon crystals clusters anchored at the surface. With zirconium-based frit, well-dispersed acicular crystals are obtained, generally aligned with the surface.
They do seem to say frits can produce longer crystals that align with the surface. I would like to guess that could make it better but I have no idea.
I have always wanted to try fritting some high fire glazes and then comparing them to the raw glazes, it would be an interesting experiment with a white glaze. Might get back to this in 10 years.
-
You could try a Zircon Frit.
-
It could be the bentonite just not everything seems to be adding up. I would think a little bentonite should help the glaze harden and not flake and crack which seems to be the opposite here. I also don't remember 2% bentonite having that much of a change to the flocculation in terms of how thick it seems but helps keep everything in suspension for longer. It has been a while since I used bentonite in a glaze, I much prefer epsom salts.
I have stored mine for at least 4 months without it seeming to change, it did get a little cloudy but still deflocculated my slip fine. I would still keep most of it undiluted and make up 5-10g of darvan dilution at a time.
-
I think Bill was talking in dry glaze, if you want to use wet glazes...
DD = Dry density g/cm3
SG = Wet density (specific gravity) g/cm3(also known as ml)
Percentage dry content = (DD / (DD - 1)) x ((SG - 1) / SG)
You can work out what the dry density of glaze is but most people use 2.6. I have written it out with brackets but you can just type it in one go without them into phone calculator.
An example would be 2.6/1.6x0.5/1.5 if you measured the specific gravity of your glaze slip to be 1.5g/ml.
That gives you 0.542 or 54.2% dry content.
To weigh out 100g of dry glaze you will need 100/0.542 = 184.5g of wet glaze if the SG is 1.5g/ml
-
If you struggle with over flocculating you can always dilute it say 4 to 1, water to darvan. Makes it easier to add slowly.
I don't really feel like 2% bentonite should cause any issues with the glaze. Are you sure you didn't add 10x too much zinc oxide when bulking up the recipe? Maybe my gut instinct is wrong and you do say it fired fine the first dip straight after mixing. If it is the bentonite epsom salts are probably a better option to help with suspension. There is a little bit of clay (kaolin) in your recipe.
It's interesting that everybody seems to find flocculated things dry slower as it should be the other way around.
-
-
-
Don't really have anything I can recommend but I think underglaze will still give you the same dark, mid and light tones you are looking for.
As far as I can tell, stains are metal oxides that have been sintered with clay or some other things to make the metal oxides more stable and consistent and underglazes are stains mixed with clay, gums, frits and so on to make them nicer to use and stick to the pot better when unfired and fired. When they are talking about adding frits it's generally to help the stain/underglaze melt as they can be quite refractory, being more refractory than the base oxides is the main reason for the underglaze suggestion. You could try buying a glaze/body stain first as they should be the most refractory but they could be too dry looking if you are painting over the glaze. Hopefully that made a little bit of sense, I am not the best at explaining.
Edit - Just read through the thread you posted and I am in it haha, it's funny remembering how little I understood when I started my journey
-
To me lower firing is certainly the easiest way to get less reaction between colour and glaze but with the wrong glaze you can still get the same look at lower temperatures too so it's not always a solution that works.
I do agree with Bill that an actual underglaze should be less reactive than pure cobalt carb/oxide and messing with the glaze chemistry (increasing silica and alumina) could also help.
Edit - just had a quick search about Portuguese Azulejos and they seem to be around the 1020c mark too.
-
Delft seem to be earthenware, same as a lot of the spode italian stuff. They are low fired to about 1025c-1075c (1875f - 1970f) which stops any bleeding into the glaze and becoming blurred. If they are porcelain or bonechina I would guess they do a high bisque and then because the colours are oil based it doesn't need to be porous to apply the artwork or the art is overglazed after the glaze fire. All the spode blue italian I used to print was onto bisque earthenware, glazed and fired to 1050c.
Not sure how they originally did it in china on high fire porcelain but some have the blurred look.
Edit - I will add when they messed up the glazing and would respray and refire 2 or 3 times the artwork would start to run and blur, especially on the more vertical surfaces.
-
16 hours ago, Bill Kielb said:
Seems pretty well filled, any sense of the r value walls, top, bottom. Seems likely double of the standard home kiln.
No idea on an r value but I just measured the walls and they are all 15cm/6inch thick
-
I remembered I had some data from kilns at work and the big one cools 100c in the first hour and the small one 220c. Shows how bad mine is at holding heat
Not sure on the size of the big one but it's about 2mx2mx2m ( maybe more 1.2m square after looking at the picture as the tiles are 10cm ). Only dropping from 1040c
Big kiln
-
There's no "soft hold" when you turn the kiln off. It will drop the fastest from highest temps and all graphs will take this sort of shape once you turn them off, besides the part where I open it before room temp. Dropping 400c in 1 hour is probably in the top of the range but I would guess most big/better insulated pottery kilns still do 250-300c in the first hour.
-
How convinced are you to work at cone 6? I have one that works pretty well at cone 03, not that my brush detail is intricate on this test but it doesn't spread. Red iron oxide mixed with water.
Low Expansion Frit - 43
Ferro Frit 3195 - 43
Kaolin - 14
https://glazy.org/recipes/440793
-
The bold heading is your results of 0.207 micrograms per milliliter and if you multiply the total volume of leachate (acetic acid) 192 x 0.207 you get 39.744 or 40 micrograms total. The reporting limit of 0.005 is the lowest values they can accurately report.
In this they say there's no legal limit for copper but 1.3 milligrams per liter is safe for drinking water and you have 207 micrograms per liter or 0.207 milligrams so about 15% of what can be inferred as "safe"
https://ceramicartsnetwork.org/ceramic-recipes/recipe/Copper-Oxide#
-
It's an interesting theory although I would think that in the high Ca, Mg and Sr glazes they will also be pulling silica out the melt when making crystals but maybe the extra charge on the Al is taking out more silica.
The matt debate has been going on for a long time. You might be interested in this thread but I wouldn't put too much thought into them talking about oxygen ratio as I have come to the conclusion that it's not much use.
-
Is there a pattern to when it cuts off or just at random temperatures?
Not sure on the size of the big one but it's about 2mx2mx2m ( maybe more 1.2m square after looking at the picture as the tiles are 10cm ). Only dropping from 1040c
Glaze firing schedule
in Clay and Glaze Chemistry
Posted
Can you use a different clay or glaze? Firing hotter isn't the best way to solve crazing.
998c looks like cone 06 at 60c an hour for last 100c of firing so for cone 04 your just need the last 960-1060 to take 100 minutes.
A standard firing would be 9 hours to 960c then 1 hour 40 min to 1060. 100c per hour climb will be slow enough for almost anything. Depending on what you make you can go 150c, 200c or 250c but 100c is a good starting place.