Clay body too prorous for glossy glaze

[Cristobal]

It's my first post here, so saying good afternoon to everyone.

I'm trying to glaze bricks for some custom masonry project for my house.
Because I'm off-grid I'm using a gas kiln that I converted from an electric kiln.
I set it up to have three shelves (each one made of two half shelves). The difference between bottom and top temperatures is around 2 cones.
I was using Spectrum low fire opaque glazes but then switched to custom made low fire glazes that I'm currently experimenting with. SPetrum looked too plastic-like to me.
I'm glazing two types of bricks:
-low duty, extruded firebricks - they glaze pretty well in a very wide range of temperatures
-hand made bricks from Spain, made of real terracota - they were originally fired in wood kiln and have great variety of shades (firing temperatures)

The Spanish bricks are the reason for writing this post.
I only managed to get almost acceptable results once, using Spectrum glaze at 05.
In all other cases: 4 different glazes and different firing temperatures: from 04 to 07 left the glossy glaze matte or absorbed in the brick body. They seem to increase their porosity while in the kiln. The bottoms of the bricks absorbed my kiln wash on the bottom (hottest) shelves. The glaze seems to be absorbed by the brick, leaving only very thin matte layer. The same conditions create proper glossy glaze on firebricks.

Do you think it's the specific chemistry of the clay (high calcium contents) or the physical structure (porosity) of the brick (they are hand made by filling the form, not by extruder) or maybe both?
Should I apply thicker layers (I'm applying by brush, but will switch to dipping) or use something that would lower their porosity or block the transfer of glaze to the brick body, or something else?

Thank you!

[Rae Reich]

An interesting project.
Maybe a slip or underglaze (maybe fired first, then gloss glaze fired). I would go with the lowest maturing temperature of the glaze since the brick is already sufficiently fired.

Hope you have plenty of extras to experiment with. 

[Cristobal]

Thank you Rae.

Since the glaze behaved as underglaze and left matte finish, I will try to reglaze the test pieces and fire again. Before I was just cutting off the glazed layer with diamond blade and glazed again.

What slip would you recommend? Some low fire clay mixed with water?

I just re-realized how porous these bricks are - hundreds of small openings visible to the naked eye and probably thousands not visible down to micron scale.

[Rae Reich]

2 hours ago, Cristobal said:

Some low fire clay mixed with water?

That would probably work. You want a smooth solid coverage where the glaze will go and maybe firing it on first will help. If everything is melting into the brick, could you be firing too high?

Since their bottoms melted the shelf wash, you might put them up above the shelf with supports so they heat evenly without extra heat  transfer from the shelf. 

[Cristobal]

The first time I fired them, the lowest shelf got 03/04 and they got a little bit fused to the shelf and the bottoms of the samples got bowed. Then I started using kiln wash and never exceeded 04 on the bottom. I decided to find a low fire glaze, to not stress the bricks, My last firing was 06 on the bottom, 07 middle and 08 top. Firebricks finally got glazed beautifully on all shelves, but Spanish samples absorbed all glaze at all temperatures. Only 08 left a small puddle of cobalt blue. It made me realize that no matter what temperature - once the glaze becomes fluid it just gets absorbed.

I will try to reglaze existing samples and also make some slip from some red clay that I have and bisque fire them first. If this works, I will get some more compatible terracota material from Europe for the slip.

[Babs]

Because if shrinkage any slip you apply needs to be carefully made and tested.

Using mininal watwr and getting tge necessary fluidity by using sodium silicate or Darvan.... experts may join the discussion here. Dont know where you pla  to use tgese bricks but being so pirous may mean issues if in the weather or where they need cleaning.....

[Rae Reich]

Maybe you just need to go to ^018 enamels for the Spanish bricks. They do not seem to do what you want at glazing temperatures.

[Cristobal]

Babs,

Today I will try the basic red clay slip and if it works I will try to get original clay that was used to make the bricks. As far as bricks are concerned - they are the most gorgeous bricks I ever saw - with multitude of colors from straw, peach and pink and imperfection of shapes - each one looks different. They are wood fired and are made with the same techniques like their ancestors hundreds years before. They are approved for renovating architectural monuments in Spain - several hundreds years old buildings. All other bricks compared to them look like made of plastic by a computer - especially all new bricks. I used them to build structural arches,, cornices in my house and other structures.. Their porosity shows only when they are cut, and I needed to cut them before enameling, because their outside surface is uneven, bumpy and contains sand particles (from the form in which they were manually formed). I will be building kitchen cabinets and I want the fronts of the bricks to be glazed, so it will be easier to clean them in the future once they get stained.

[Babs]

Slate sealers instead of glaze?

[PeterH]

On 7/30/2024 at 7:21 PM, Cristobal said:

They seem to increase their porosity while in the kiln. The bottoms of the bricks absorbed my kiln wash on the bottom (hottest) shelves.

Does anybody have an explanation for that? Isn't kiln wash is supposed to be refractory, so it shouldn't be absorbed. Unless the brick is softening sufficiently for the kiln wash to be simply stuck onto it.

What happens if you adopt Rae's suggestion of supporting the bricks off the shelf?  

On 7/30/2024 at 7:21 PM, Cristobal said:

The glaze seems to be absorbed by the brick, leaving only very thin matte layer. The same conditions create proper glossy glaze on firebricks.

Random thoughts.

1) If all else fails might a brick sealant be an alternative?

2) Would adding gum to the glaze manage to keep more of the unfired glaze ingredients on top of the highly porous surface of the brick? Hoping that the high viscosity of the melted glaze will then keep it at the surface during the  firing.

PS Terracotta might not be the easiest body to glaze, especially when not chosen/fired for pottery purposes.

Bubbles in Terra Cotta transparent glazes. What to do?
https://insight-live.com/show_post.php?pz=duwdejucaz

Terra Cotta
https://digitalfire.com/glossary/terra+cotta
... perhaps especially How to decide what temperature to fire this terra cotta at?

 

[Rae Reich]

@Cristobal, I love your description of the bricks, their colors and their uniqueness. Therefore, I think you should quit trying to glaze them. Instead, use a time-tested technique of waxing to seal them.. Others may know the terms or brands but there’s a hard-sealing wax (commercial grade?) that is used for sealing brick and paving tile. The colors will deepen, but remain spectacular. And easy to mop or wipe down.

Edited August 2 by Rae Reich

[Cristobal]

I'm just waiting for the kiln to cool. I reglazed the bricks that absorbed the glaze and painted some fresh ones with the simple red clay slip I prepared. The firing finished at 1 AM. I will open it Saturday morning.

In reply to PeterH:

When I used Spectrum Opaque Low Fire glaze and someone fired them for me in an electric kiln, actually they did not absorb the glaze and the results were quite satisfactory (despite the fact that opaque glazes look plasticy to me). So it looks that the gum in the commercial glaze and slow firing did help. Then I used a pottery studio firing services and they could not properly fire the enamel - it was always underfired. Then I realized that I can not rely on external services and decided to do kiln conversion, because I need to glaze around 350 bricks and more in the future. The firings I'm doing are rather "wild": 5-7 hours full firing, but I like it.  Terracota is/was traditionally glazed in wood fired kilns that are probably even more difficult to control, but at the same time the temperature rises slower that in my gas kiln.

I would not use a sealant, I will probably use linseed oil on my terracota floor that is smooth and made from the same material.  I'm accepting possibility that I will not glaze the bricks the way I want and I still will be happy, but wanted to do something different and it pushed me into the rabbit hole of ceramics, which I always wanted to explore anyways.

Interesting thing is that the manufacturer of these bricks was not able to enamel them, because they are too grainy on the outside - he was not obviously cutting them. On the other hand, he is selling glazed version of the rooftiles, but the rooftiles are smooth, better compacted and harder than the bricks. I need to cut them with the continuous rim blade and water and brick I can easily dry cut with regular segmented diamond blade.

[PeterH]

Loosely related
Outdoor Weather Resistant Ceramics
https://digitalfire.com/article/outdoor+weather+resistant+ceramics
... including a picture of a terracotta tile half coated with a sealer.

I've no idea how these sealers stand up to repeated cleaning.  I wonder if any of the anti-graffiti sealers are suitable.

Edited August 2 by PeterH

[PeterH]

@Prav Any thoughts? (I've tried to PM him.)

PS I wonder how much the thermal inertia of the bricks influences the firing. Do you need to fire very slowly for the surface of the brick to reach the indicated cone?

[Cristobal]

I just opened the kiln.
Bricks that I reglazed got a little bit more glossiness in spots, but still completely unacceptable in every way.
The slip on bricks did not work. The bottom shelf reached cone 05 and the slip is peeling off - probably because of different expansion ratio than the body of brick, so either I get some of the original clay from the manufacturer or will have to abandon this project. It's already going for over a year and I can do it only in the dry season, because my entire "workshop" is outside.
Even with the slip made from the same clay it will not guarantee a success, but at least it would fill the visible pores in the brick and I would deal with just a regular micro porosity of the material.

 

I'm attaching 4 images:

Samples are around 35x50 mm. I reglazed them with dipping. The slip was applied by brush.

From top to bottom: shelf 3, shelf 2, shelf 1 (hottest), shelf 1 with slip:

Edited August 3 by Cristobal

[Babs]

5 hours ago, Cristobal said:

I just opened the kiln.
Bricks that I reglazed got a little bit more glossiness in spots, but still completely unacceptable in every way.
The slip on bricks did not work. The bottom shelf reached cone 05 and the slip is peeling off - probably because of different expansion ratio than the body of brick, so either I get some of the original clay from the manufacturer or will have to abandon this project. It's already going for over a year and I can do it only in the dry season, because my entire "workshop" is outside.
Even with the slip made from the same clay it will not guarantee a success, but at least it would fill the visible pores in the brick and I would deal with just a regular micro porosity of the material.

 

I'm attaching 4 images:

Samples are around 35x50 mm. I reglazed them with dipping. The slip was applied by brush.

From top to bottom: shelf 3, shelf 2, shelf 1 (hottest), shelf 1 with slip:

Slip did what I expected if not made by greatly reducing the amount if water to get the shrinkage right.

LivIng with a pantry with painted rough  hand hewn rock, with low splash back of wood, and the impossible task of keeping it clean for 40yrs, don't use these bricks in an area which has heavy hand traffic, splashes etc and so  requires a thorough cleaning. A filler, then paint would be the only way I'd consider it.

Your beautiful terracotta ones I'd be oil/waxing them as others suggest.

[Rae Reich]

On 7/30/2024 at 12:21 PM, Cristobal said:

was using Spectrum low fire opaque glazes but then switched to custom made low fire glazes that I'm currently experimenting with. SPetrum looked too plastic-like to me.

Are your fired samples the ‘custom made low fire glazes’ above?  Were they intended for dipping or brushing?  They look like they have not been applied thickly enough. The white glaze,  especially, needs more coats. Dipping glazes require absorption, not just porosity. You would need to reduce the water in a dipping glaze and add a thickener, as Babs described above,  to make them adhere properly. That can be done with some study and experimentation.

The Spectrum glazes and many others like them are formulated to be applied by brushing and their composition is probably more compatible with the porosity/lack of absorption of your brick. If you can thicken up your custom glazes to act the same way,  you might apply the glazes to many bricks at once with a paint roller.

The amount of temperature variance between your shelves will not give you consistency with any glazes that I know of. Your best hope is to find some that look acceptable to you at the three stages of maturity OR work on learning to bring your kiln to more similarity from top to bottom.

 

Edited August 4 by Rae Reich

[Cristobal]

Rae,

I always applied three brush coats of Spectrum - the were absorbed the same way in my kiln - only once on top shelf at cone 05 they could be considered acceptable.

The custom glaze I'm using is cone 08 borate-based and firing beautifully on firebricks on all shelves. I applied it by brushing and that was the first glaze that made me happy (after I tried three other ones, - frit-based). When I dipped  them they also turned out beautiful between 05 and 08. Glaze density was around 1.7. I love this glaze, because it seems compatible with my rough conditions and the glaze works for me and not the opposite. For all things that I do (especially for gardening and farming), I'm always trying to find the limit "how much to baby things", but at the same time the act of babying things creates true masters...

 

Babs,

I'm considering to leave the Spanish bricks as they are, just seal/wax them. I just wanted to do something different with them, but maybe it was too pretentious anyway.

[Min]

I would hazard a guess that the bricks in the link Peter supplied were either a less porous material than those in this thread or there has been an application of an engobe applied to the surface of the bricks prior to bisque firing.

[Cristobal]

Min,

The picture of the bricks in the link posted by Peter shows bricks with holes, so definitely machine made. The commercial clay pugmill probably has vacuum pump to remove any bubbles. Machine made, extruded firebricks glaze normally.

I would like to mention zellige tiles - also made from terracota and their terracota looks very similar to my bricks with the same color variation and hardness when I cut them. It's possible that their clay was created by the same geological processes, from the same deposits in the similar climatic zone as Andalucia across the sea. They glaze beautifully - the artisans pound leather hard clay tile flattening it to the desired thickness and then they SMOOTH it with a tool, to close all macro pores.