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preeta

Can I Use This ^10 Slip As A ^5 Slip?

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this is a Akira Satake recipe that Bruce posted in a previous thread. Can i use this recipe for ^5 or do i need to change anything? Goldart? How would I change this recipe to suit a ^5 claybody and firing.

 

First, here is Akira's slip recipe: 

Goldart, 6 lbs. (30%)

Kaolin - EPK, 10 lbs. (50%) [Akira also uses Grolleg or Tile 6 for a whiter slip and Helmar for woodfired items]

Custer Feldspar, 2 lbs. (10%)

Silica, 2 lbs. (10%)

 

Thanks

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You should try it just to see what happens. If there is a crazing or flaking issue it might mean that the clay and slip have incomparable shrinking rates, and if that's the problem, try mixing a walnut size piece of clay in with the slip. That seems to make the bond stronger and the shrinkage rate similar.

Be sure to make written notes.

Keep us posted.

 

Alabama

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I use the slip at cone 6 and cone 10 (just finished applying it to some Little Loafers slabs for cone 6). I had a couple pieces flake on two or three pieces done in Red Rock -- a stoneware that shrinks a bit more than Little Loafers. Those could have been the result of texturing the slabs, too, I apply just after rolling slabs when the clay is rather moist. Have not tried it a cone 5.

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thanks Bruce for posting that thread. it was VERY helpful. i learnt so much and cant wait to try the technique.

 

i'll keep you posted when i lay my hands on some Goldart if i can find a local source. alabama thanks for the reminder, I will record everything. 

 

bruce i have found if i apply the slip at the clays really moist state it really sticks well. it does have a different look especially when slip trailing. 

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Thoughts:

1. If this recipe was formulated before the big changes in custer, there could be a problem. The old blend ran 10% potassium, and the new is running between 5 to 7% potassium on any given day.

2. This recipe runs short on flux & silica at cone 6.

Nerd

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dick white - that is true. but this semester while playing with a slip i found it really crumbled and came off in great amounts when i took it to cone 5. it was a low fire slip (if i remember right cone 1). yet it did great at ^04

 

aha nerd. just what i was looking for. but will silica still matter in a slip?

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Preeta, an important part of both slip and glaze fit is matching its shrinkage to that of the clay body. For glaze, it is the thermal shrinkage that occurs during cooling. For slip, it is the shrinkage that occurs as a consequence of vitrification. If you were to make some shrinkage rulers with mid- or high-fire clay (strips of your clay with measured marks applied just after rolling out the slab), you would find that about half the total shrinkage (6-7%) occurs when drying to bone dry and the remaining half of the total shrinkage (now totaling 12-13%) occurs in the glaze firing as the body achieves vitrification. The difference in total shrinkage between a mid-fire (cone 6) and high fire (cone 10) body is generally small, only about 1% more for the high fired item. A compatible slip will match this.

 

Conversely, a low-fire clay body never goes past the first stage of shrinkage (~6%), as it doesn't vitrify. Thus, a low-fire slip need not shrink for vitrification either. But if you put it on a mid-fire clay body and fire it to vitrification, there will a significant mis-match because of the second phase of shrinkage during vitrification and the slip would likely shear off. I suspect this is what happened when you fired the cone 1 slip to cone 5. For this kohiki slip, the shrinkage variance between cone 6 and 10 is small and furthermore, it is intended to fracture.

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Preeta:

Clay/slip has formulation criteria and limits, just as glaze does ( but different). This recipe would actually be classified as a refractory slip because of the high alumina. It was designed to have lowered silica. Someone did some careful planning when they developed it: bravo! However, when it is fired to cone 6 there is just barely enough silica, and not enough flux to develop. 

Nerd

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ooh thank you both for your explanations. this is exactly why i love this field, it meets both my intellectual needs as well as creative and then throw in the unpredictable character.

 

i am trying to up my knowledge about basic principles of glazing. can you refer some basic bibles to read on that. right now i am reading my first glaze book - Pottery Glazes by David Green. while it is outdated, i am discovering so much. i do have a basic geology and chemistry background.

 

glazenerd why did the big change in custer happen? did some ingredients price go up or are they mining it from a different place? 

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Glad we can help you along the path to perdition in this life :) . You asked about some books, for which I have 3 recommendations and others may offer their favorites. A current (publ. 2014) and very thorough book on mid-fire glazes is the new John Britt book "The Complete Guide to Mid-Range Glazes." Not only does it have one of the most complete compendiums of glaze recipes, he offers considerable information about raw materials and techniques. Another must-have book (IMO) if you wish to fully understand the deep down in underpinnings of pottery generally is Frank and Janet Hamer's "The Potter's Dictionary of Materials and Techniques." Finally, you mention having a background in geology and chemistry. If you want to take that to its ultimate destination, Mimi Obstler's book "Out of the Earth Into The Fire" will take you there.

 

As for the Custer feldspar fiasco, nobody knows the truth as Pacer (the company that processes and sells it) won't own up to it. The whole "industry" of potash feldspars in the USA has been in transition for many decades. Long ago there were several purveyors, Kingman, Buckingham, Clinchfield, G200, and Custer being the big players. The Kingman, Buckingham, and Clinchfield mines were exhausted a long time ago (but I have some that I have collected from here and there over the years...), leaving G200 and Custer. They were similar enough that usually could be substituted directly. Then the main G200 mine ran out, but they had another mine with a feldspar of much higher potassium content. But, by mixing it with some soda feldspar, they could replicate the original chemistry. Then trucking costs went sky-high and they stopped bringing the soda spar in from 3 states away. Instead they relabeled the bags and started selling the high potash spar as G200HP, including appropriate corporate communications to the marketplace of the change and instruction for blending with your own soda spar to get back to the original if that's what you need. Sadly, that mine too ran out a few years ago. Which leaves us now with Custer as the only domestic source of potash spar. But we potters are only a minuscule part of their market. Industrial users (principally abrasives and paint fillers) are their bread and butter. As you may have learned, the chemical makeup of our various glaze materials is important to know when formulating or adjusting a glaze or clay recipe, and the sources of these materials usually publish a technical sheet giving a typical analysis of their product. Pacer has such a sheet for Custer, but the analysis data is unchanged from at least a decade ago. However, about 5 years ago, many potters began noticing that glazes containing significant proportions of Custer were coming out crusty and underfired. Nobody could figure it out, as Pacer's published analysis was unchanged. Finally, a few potters with access to the appropriate resources sent some samples from  their latest purchases to independent labs for technical analysis, and the new stuff came up short. As Glazenerd notes, the old standard was 10%K while the new stuff is coming in around 7%. To make things worse, Al is also down from 17% to 15% and Si is up from 69% to 74%. That can make a serious problem in a glaze that has a significant proportion of Custer as its flux. However, Pacer still insists their current product conforms to the old analysis, and if you were to directly challenge their corporate mouthpiece (as some potters have done) you would be blown off as just another stupid potter. To top it all off, Pacer used to sell two grades of the product - 200 mesh and 325 mesh. Last winter, their 200 mesh grinder bit the dust and they made a corporate decision to not repair it. At least they publicly announced that 200 mesh would no longer be available and users should adjust their expectations as needed to continue with the 325 mesh product. So that's the long story. The short story is you are on your own with recipes containing Custer.

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oh THANK YOU dick for all the explanation. i really appreciate your time. politics everywhere. yeah in the whole realm of that world i do understand potters and ceramicists are a tiny dot yet they are pulled up for not being green enough and the big companies go scott-free.

 

thanks for the book recommendation. i do have Hamer's book, and Britt is on my wish list. i'll add obstler's book on my wish list too. 

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ok ive used  porcelain slip (high fire)  on non porcelain bodies @ cone 6 no issues in fit..... (but the clay gods have been lenient with me)

 

thing is i think since you are adding it to "wetish".... you get forgiveness factor....   the kohiki finish is a bit "dry" so i think you will get away with same recipe.

 

i think you'll get a closer cone 5 recipe subbing nephsy  for the custer

 

i did make a few  kohiki test tiles with unknown studio white clay slip  over standard 266  with promising results but never revisited....

 

warning the above statements have no science behind other than some empirical results....

 

i'd me more concerned with getting a well behaved cone 5 clay that is dark enough......

 

it is possible to re slip/wash over with darker slip/wash  then wipe back to white.......

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Dick: big amen on Hamers' book- text book classic.

DigitialFire is okay, but I use other resources.

 

Preeta: the geological term is strata, what most call veining in the earth of various materials. Ever see pictures of mountains that look like there are layers of colors? Same thing happens in the ground: mines dig through layers of strata as they go. In doing so, they hit various deposits that have differing chemical composition, even though they are the same material.

 

The clay biz is turning more and more to Nep Sy because of price, but more so because of consistency.

Nerd

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golly! i am seeing more and more of a link between glazes and glass blowing. 

 

wonder if both have come together ever in a big way! i see big potential but i have not come across a big experiment in my limited search and reading. 

 

yeah digitalfire is my pinterest. it is at least a starting point. and when they explain why i am in heaven! however sometimes i dont find stuff there that others do.

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