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Lichen Glaze "peels" Off When Applied


SAS Pottery

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It's been a while since I've played around with a Lichen glaze and now remember about it "peeling off" my base glaze when I applied it. I sprayed on a matt black slip base glaze - waited for it to "dry" but not completely, then did a pour of the Lichen glaze around the neck of the pot - the Lichen glaze poured down the sides well but within 5 mins, it began to "peel off" the entire piece. I had to wash off the entire piece and will start from scratch again once it's dry.

 

For those that do Lichen based glazes, thoughts/techniques to keep it onto the surface base glaze better? I've read about it being "brushed" on or "sprayed on" - but I like the poured on effect look. Does the Lichen glaze adhere better if the base glaze is completely dry, somewhat dry or just after it's applied? How have Forum members "overcome" the peeling problem?

 

Thanks - Scott

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SAS, 

The first link is to Robin Hopper's article on Ceramic Arts Daily (see upper left hand header to access the CAD

 

 

 

http://ceramicartsdaily.org/ceramic-glaze-recipes/glaze-chemistry-ceramic-glaze-recipes-2/lichens-and-lizards-and-leopards-oh-my-reticulated-glaze-recipes-for-wild-ceramic-surfaces/

 

Next is Alisa Clausen's post from Clayart regarding mark Bell's Lichen which is in Lana Wilson's book. Alisa is the only one who mentions application requirements. Possibly there are dome more directions in Lana Wilson's book.

 

Lichen Glazes are fun to use but tricky.You could google "applying lichen glazes for more information.

Marcia

 

Alisa Clausen on mon 21 feb 05


Glaze test for Mark Bell's Lichen Glaze, cone 6 ox. 

Glaze tested on white midrange stoneware fired in electric oxidation to = 

1240c. (2264f) 


Source: 

Credited to: Mark Bell 


Firing ramp: 

100c p/h to 600c (212f - 1112f) 

150c p/h to 1140c (302f - 2084f)=20 

80c p/h to 1240c (176f - 2264f) 

10 min. soak 

cool down max. to 1100c (2012f) 

Hold 1 hour 

Shut off kiln 


Recipe: 

30 Soda Spar 

31 Magnesium Carb. 

6 Frit 25 

8 Talc 

6 Zinc Oxide 

10 Kaolin 


Note: All raw materials are measured up or down to the nearest half = 

decimal. =20 

Colorants or additives to a 100 gram test batch are measured in percent = 

to the 100 gram test batch. 


Results: 

The application of this glaze is crucial. It must be applied thickly = 

enough that it dries with slight cracks over the dried surface. 


This recipe is very similar to Brian Kemp's recipe, with the addition of = 

Zinc Oxide.=20 

The glaze forms small, close, irregularly shaped curdles. They are have = 

a more mat surface than the Kemp recipe. The addition=20 

of Zinc also makes the overall glaze slightly drier and the edges of the = 

curdles are slightly lifted. The overall feel is craggy and rough. 

It resembles lichen moss and although the edges are lifted, they are = 

fused. 


With 3% added Rutile 

Same as above, but slightly larger lichen forms, color is also the same. 


Over Commercial Black cone 04 glaze 

the curdles smoothed out, are in general larger and glossier. The Black = 

glaze pools in between the curdles. 


Regards from Alisa in Denmark 

 

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I'm not so sure putting a crawl over a slip glaze is going to fire okay. I've played around with crawls a bit and when I used them over underglazes some colours had enough flux in them to cause the crawls to turn to blobs.

 

I've tried them over velvets, spectrum and a few of Seattle Pottery Supply ugs, most colours worked, but if the ug looks shiny when fired (cone 6) then the tests with crawls overtop tended to be less crawl and more icky blob look. I think that if you use a slip glaze the crawl is going to melt into the glaze and be more along the blob lines than a crawl.

 

Since the crawl glazes like to peel off the pot even without raw glaze underneath I can see it would be a problem getting it on thick enough without peeling/shelling off with a raw slip glaze underneath.

 

Ikebana with spectrum tan ug (on greenware then bisqued), under sprayed crawl. (let me know if you want my ^6 crawl glaze recipe and I'll dig it out)

 

edit: Hopper is using a slip not a slip glaze as in your post. Try searching crawl and reticulated glazes too.

post-747-0-03154800-1440425949_thumb.jpg

post-747-0-03154800-1440425949_thumb.jpg

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Marcia - did read Robin's article prior to my first post but he doesn't address my original problem of the Lichen peeling off if it is poured vs. brushing or spraying it on - I have done this process of pouring a few years ago and got the same "peeling off" problem but I then did it again and it stayed on. It cracked as it was going to anyway but stayed on the pot (the pot was spherical) and fired out really nice with no "flaking" issues in the firing. Trying it again now and the above post results are as I've mentioned.

 

I did do an initial "using Lichen glazes" but not the word "applying" in my original Google search. I'll see if I can get other "hits" with that "applying" word.

 

I'll also try to find the Clay Art posting you mentioned to see what else she has to say - applying it on "thick" may work - I have the Lichen glaze mixed as if it were a "normal" glaze to dip or pour - perhaps for its purpose, it does need a "thicker" consistency as if it were a slip.

 

Other thoughts from members?

 

Scott

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http://digitalfire.com/4sight/troubleshooting/ceramic_troubleshooting_powdering_cracking_and_settling_glazes.html

 

 

 

As you can see, we want a glaze to have enough clay so that it forms a hard dry layer but not so clay that it shrinks excessively and cracks off the bisque. There are a number of strategies you can employ if your glaze is powdering on one extreme or shrinking and cracking off on the other. It follows that a powdering glaze needs either more clay or a finer more plastic clay whereas a glaze that is shrinking and cracking needs less clay or less plastic clay. Typically pottery glazes need a minimum of 15% kaolin to harden adequately. Ball clays and bentonites can be used, as well as other clay materials, but kaolin works the best to gel the slurry. It might seem that because ball clay is much more plastic than kaolin, you could use a lot less, but in practice, 15% ball is also needed. The same can be said for bentonite, while 5% bentonite might plasticize a body as much as 15% kaolin, this alone is not enough to harden a glaze well, the bulk is needed.

 

I would have thought there is a lot of clay in the recipe but that doesn't seem the case if you are using the one posted above. From my tests I know high clay or high zinc content can make glaze crawl. Doesn't seem to be much in it. Looks like something is shrinking too much and making glaze pop off, if not the clay, possible mag carb but I haven't used that before.

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I remember seeing a demo on 2-part raku smoke resist by Gabriela Koch many years ago,

applying the two layers by by dipping. Following a question from a member of the audience

she said that the top layer flaked off unless it was applied

- either less than about 20 mins (or was it 40?) after the first

- or after 18hrs or so

 

It just might be worth doing a test applying the lichen glaze after 18hrs or so.

 

PS I never found a rational explanation for Koch's experiences.

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I remember seeing a demo on 2-part raku smoke resist by Gabriela Koch many years ago,

applying the two layers by by dipping. Following a question from a member of the audience

she said that the top layer flaked off unless it was applied

- either less than about 20 mins (or was it 40?) after the first

- or after 18hrs or so

 

It just might be worth doing a test applying the lichen glaze after 18hrs or so.

 

PS I never found a rational explanation for Koch's experiences.

 

The moisture in the first layer of glaze/slip and the body is key here. You either apply the next coat while the under layers have enough moisture that the two dry together, or you wait for the first to dry, so the rehydration of the under layers doesn't push it off of the body (notice how clay rehydrates well when wet or dry, but is resistant to moisture in the leather hard stages, similar interaction).

 

The biggest adjustment to make with these crawl glazes is the amount of mag carb. That's the material with the super high CoE doing all the crawling work. Changing the amount can change the amount and size of crawling, and by reducing the amount of fluxes, also changes the melt of the crawl (You can get crawls that look sharp like peeling layers of slip, versus melted blobs of glaze). But, as suggested, try application of the top layer fairly soon after the base, and also try it after allowing the base to dry to see how it responds.

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I'll also try to find the Clay Art posting you mentioned to see what else she has to say - applying it on "thick" may work - I have the Lichen glaze mixed as if it were a "normal" glaze to dip or pour - perhaps for its purpose, it does need a "thicker" consistency as if it were a slip.

 

Scott

 

needs to be as thick as runny pancake batter, much thicker than average 1.4 sg of glaze

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The moisture in the first layer of glaze/slip and the body is key here. You either apply the next coat while the under layers have enough moisture that the two dry together, or you wait for the first to dry, so the rehydration of the under layers doesn't push it off of the body (notice how clay rehydrates well when wet or dry, but is resistant to moisture in the leather hard stages, similar interaction).

 

Thank you, that makes a lot of sense. Peter

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