Japanese glaze question

[Anatol]

I brought this piece some time ago from Japan and now  trying to figure  how it was made. What is this  black speckled glaze? Are there any commercial analogues? Is it simply granulate added to a metallic glaze, or perhaps tenmoku glaze? And the top white glaze, was it fired to lower cone then underlying glaze? The two stay separate and don’t blend,  while copper (?) running from the metallic base giving this bluish tint...

[liambesaw]

Looks like a traditional oxide based black (cobalt + iron) fired in reduction.  The white kinda looks like a nuka glaze.  They're fired at the same time. The reduction can bring out some metallic sheen from the oxides, which is the metallic brown you see on the bottom with the darker spots probably being the cobalt.

[Mark C.]

My guess is the outside glaze is a high manganese content glaze-looks like one i use.Tom Coleman published the recipes in glaze I use.

[Anatol]

@liambesaw thanks for the answer!  So if I take any transparent glaze for the right cone, add 5% (?) of cobalt oxide and 5%(?) of iron oxide and then fire in reduction I suppose to get this speckled effect?  I just love this black speckles effect on black grayish surface Which cone is it?

[Anatol]

@Mark C.thanks for the answer!  Is this recipe available somewhere online, or print only? 

[Mark C.]

Looks like a cone 10 glaze outside and inside glazes separately .

[neilestrick]

On 9/7/2019 at 3:48 AM, Anatol said:

@liambesaw thanks for the answer!  So if I take any transparent glaze for the right cone, add 5% (?) of cobalt oxide and 5%(?) of iron oxide and then fire in reduction I suppose to get this speckled effect?  I just love this black speckles effect on black grayish surface Which cone is it?

Nope. It's not that simple at all. I don't know what the dark glaze is, or what they added for the speckles. But it's not just a transparent glaze because it would be glossy if it was. It's definitely high in iron, but I'm not convinced there's any cobalt in it. The glossy glaze on the inside looks like a titanium white which has been applied over the iron glaze. That combo is what's giving the blue. It would be a different blue if there was cobalt in the dark glaze.

5% cobalt is way too much for any glaze. Find a good book on glaze formulation before you jump into mixing your own. It's important to make sure your glazes are safe. Coloring oxides don't all work at the same percentages. Some require more, some less. They all have different maximums that are considered safe.

[JohnnyK]

The black speckles on the brown could also be something as simple as dipping a stiff brush in a black glaze and spattering it over the brown. I've used this technique before on some of my work. It takes some practice to get the spackle pattern right, but it is doable and nothing exotic...

[Hulk]

I've found picking up liquid to be speckly with an ear syringe to be blown out in fine drops worth trying - practice in the glaze bucket first; however, likely better for speckle effect that's uneven on purpose, as in a gradient of speckles from dense to minimal.

[Anatol]

@neilestrick thanks! I found glazy.org and already see that even 1% of cobalt is already too much. Will  study this site for the start, it's the wealth of information.

[Babs]

John  Britt gets the inside effect by applying a white glaze over a glaze rich in iron.. oilspot thrn white applied thickly.

Might be what you are after.