adding granular feldspar or stone to clay body

[Giotto]

Hi - I am trying to get some information on additives to wedge into cone 6 clay to get a very textured/lumpy effect.  I read that you can add granular feldspar, flint or granite to your clay.  I am looking for a really rough texture that also has bits coming out of the clay.  If so, can you please advise on how much to use per pound of clay, as well as the firing specs?  I typically bisque to cone 04 and fire to cone 6.   

If I am way off here, and what I am suggesting will not work, if you have any other suggestions to get a very textured/lava-like look to the surface of clay prior to glazing.  Thank you in advance.

[Min]

How much is a bit like asking how long is a piece of string  As much as you need to get the texture you are looking for. Do you want to see the protrusions or do you want to leave burn out pits like from adding sawdust etc or a lumpy bumpy lava glaze? Another option would be to add feldspar chicken grit (or whatever) to a slip made from the same body as the clay and apply a layer of that to the outside of the form then sponge/wipe it back to reveal the grit.

Welcome to the forum.

[Giotto]

Thank you so much for replying and for your advise...I will try this.  I am looking for the lumpy bumpy lava glaze.  Are there any other additives you recommend.  Thank you again.... it is very much appreciated.

[Bill Kielb]

18 minutes ago, Giotto said:

.  Are there any other additives you recommend. 

A common way to get lava glazes is with silicon carbide. Here is a sample https://glazy.org/recipes/76600. If this is your intention you may want to search Glazy.org for ideas / formulations.

Edited September 5, 2022 by Bill Kielb

[PeterH]

Re lava glazes - the mesh size of the silicon carbide can influence the effect it has,
Testing Silicon Carbide Grits in Cone 6 Lava / Crater Glazes
https://www.thestudiomanager.com/posts/tag/lava+glaze

The horizontally-fired test tiles at an angle to show some of the texture. There is virtually no lava / crater effect with coarser grits.

The finer grades of SiC are often sold for grinding/stone-tumbling, rather than pottery. 

PS SiC is also used for local reduction, where the trend also seems to be towards finer grades. 
https://glazy.org/materials/15401
Note: the description mentions fine grades of silicon carbide from 450-600 mesh. Lately a number of recipes call for even finer meshes, from 1000-1200, for producing evenly locally reduced glazes like celadons. A wide range of mesh sizes can be found online from lapidary supply stores.

[Giotto]

This is great information.  Thank you SO much!

[Giotto]

Hi again - I am new to experimenting with glazes, and I have yet to make my own.  Can your information be applied to ready made glazes?  And if so, would you be able to recommend an amount to add per ounce?  Thank you in  advance, your advise and information is much appreciated.

[Min]

You could test between 1.5 and 3% silicon carbide added to commercial glaze and see what you get. Issue is we don't know how much water is in a liquid commercial glaze. To get more precise you could dry out about 200 grams of liquid glaze then weigh 100 grams of the dry powder and then add the percentage of silicon carbide and rehydrate and sieve. If you do this I would try for 100 grams of dry glaze then add 1.5% silicon carbide plus water to bring it back to brushing consistency, brush a test tile then add another 0.5% and brush another test tile then twice more to get to the 3%. Base glaze will be depleted with each test tile but this should get you some info to further refine. 

edit: forgot to add, don't fire them close to your elements or other pots, they can spit. 

Edited September 5, 2022 by Min

[Babs]

Does anyone remember folk adding burnables to glazes to get a lava effect...rice bubbles?

Might be fun to try

John Britt comes to mind.

[Pres]

Ohh Babs, like playing with bananas on a large platter, or peach skin mixed into glazes, or flower stems and flowers inside of bowls. Only in reduction, but it was fun.

 

best,

Pres

[Babs]

7 minutes ago, Pres said:

Ohh Babs, like playing with bananas on a large platter, or peach skin mixed into glazes, or flower stems and flowers inside of bowls. Only in reduction, but it was fun.

 

best,

Pres

Some absolutely beautiful results.

Did the same on fabric way efore the ecoprint bonanza.

Nothing left untouched.

Lunch boxes not even safe from mad potters 

Sorry for the sidestep folks.

[PeterH]

10 hours ago, Min said:

You could test between 1.5 and 3% silicon carbide added to commercial glaze and see what you get. Issue is we don't know how much water is in a liquid commercial glaze.

+1

Would it be easier to use Brongniart's formula?

http://www.potteryatoldtoolijooaschool.com/brongniarts_formula_made_easy.pdf

A popular way to measure specific-gravity/relative-density  uses a plastic syringe.
https://ratcitystudios.com/blog/2017/11/14/specific-gravity

PS If you cannot measure grams on your scales you will want to calculate the weight of water to fill the syringe.
https://www.rapidtables.com/convert/weight/gram-to-ounce.html

Edited September 6, 2022 by PeterH

[Min]

7 hours ago, PeterH said:

Would it be easier to use Brongniart's formula?

Good idea! Could use a Brongniart calculator to do the math. https://pietermostert.github.io/SG_calc/html/brongniart.html

Either way it's going to include the gum but since that will be present in both the sample plus the actual glaze it's a constant.

[blackthorn]

I recall seeing this woman's extensive pursuit of additions to clay bodies. Plus I did buy her book. So, maybe this will have some useful information:

https://ceramicartsnetwork.org/pottery-making-illustrated/pottery-making-illustrated-article/In-the-Studio-Supersizing-Clay-Bodies#

[Giotto]

Thank everyone who responded.  Very helpful information!

[PeterH]

On 9/6/2022 at 12:02 AM, Min said:

You could test between 1.5 and 3% silicon carbide added to commercial glaze and see what you get.

On 9/5/2022 at 10:02 PM, Giotto said:

Hi again - I am new to experimenting with glazes, and I have yet to make my own. 

Just to remind you that volumetric blending is sometimes helpful. If you wanted to make test tiles with 1.5%, 2.0%, 2.5% & 3.0% SiC you could mix up 4 glazes, or you could mix up a 3.0% glaze and "dilute" it with commercial glaze.

You can then make a smallish quantity of an x% glaze by mixing  1 volume of the 3.0% glaze with (3.0/x)-1 volumes of the commercial glaze. (I would use a small syringe).

Obviously this is an approximation, which assumes that the volume of SiC is negligible.

1.5% = 1:1, 2.0% = 1:0.5, 2.5% = 1:0.25, 3.0% = 1:0