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Might be much more successful with a body stain but could do a nu.ber of tests.

Think large amounts of iron oxide needed to change your clay colour to black will change your clay ..lower firing range . Think you might end up with a brown...

Edited by Babs
Replace "again" with stain!:-)))
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Black mason stain would likely get you closer to the colour you want.  Iron oxide’s colour can change in the firing.

There’s a thread on colouring clay with black stain around here somwhere.  Try adding incrementally larger amounts to some test batches and make some tiles to nail down how much you need.

Downside of mason stains are that they’re more expensive and the oxides used to get black are less safe than BIO alone.

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11 minutes ago, Babs said:

Perhaps sone photos will assist here Tyler.

Gee, Babs, I’m beginning to detect a little hostility.  But since you asked

Here’s an urn top I exploited iron oxide’s ability to bleed through a glaze.  10% iron oxide slip under shino.  I do this a lot.


And, here’s an undesirable one where a 10% iron oxide slip bled so bad it turned the glaze honey in spots and faded from the body:



Point made? 

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Babs, let me ask this, what would you have preferred me illustrate?


2 hours ago, Tyler Miller said:

I agree about BIO being cheaper (by a factor of five), but if colour bleed is an issue, I’d leave it out too.  Half my glaze palette relies on iron’s ability to bleed.

This is literally the only point I’ve made that could have possibly required illustration.  Which I’ve done at your request.

This is starting to feel like troll baiting, though.

I’m seriously just here to add my opinion.  I use iron slips and they bleed.  It’s a valid point if you have a glaze that likes to pick up colour.  If cobalt’s a problem, iron can be too.

I apologize to the OP.


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I think you are reading a Lot into what I wrote above.

Being a rural person don' know what you mean by troll baiting.... some folk bait  feral animals, I find that inhumane

I asked if you would post photos ..you did. I responded to that..

No personal stuff takes way too much energy. 

Cobalt will be more prone to bleed than iron, I have observed

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@Meganthwaites, there is a thread here that discusses making black slip. Chris Campbell makes lovely work with coloured porcelain, she uses Mason Best Black stain #6600. It does contain cobalt plus some other oxides. Problem with just using iron oxide is you aren't going to get black using it alone. If you look at the ingredients used in making black stains or colourants used for making a black glaze they are combinations of oxides. Cobalt plus iron and or chrome, manganese etc. If you only want to make up a little black slip then using stain would be the way I would go, if you have a large amount to make then augmenting the stain with less expensive materials like iron might be an option.

edit: like Tyler said though, using stains is considered safer for the potter than using oxides such as manganese or cobalt or chrome. (iron isn't an issue)

Edited by Min
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The photo below is the black recipe I spoke of. The piece is upside down, so you can see the black color with and without glaze. Mason Stains have a chart; only a select few are marked as " body stains."  U.S. pigments also has a select group marked as "body stains."  The 3% red iron oxide and 5% black stain is based on dry weight. So in calculating that for slip: simply deduct 40% of the weight to reach dry recipe. EX: 10lbs of slip minus 4lbs. (40% water) = 6 lbs dry material. Slips are generally 40% water. +/-  start with small batches until you get the color you are after: make notes of the additions used in each.



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