Strange round bumps in the bowl of mugs

[moh]

I'm seeing these strange round bumps appearing in the deep area of all the mugs I've been working on lately.

None of these bumps appear in vertical areas covered by the glaze. 

The glaze is sieved to 100 mesh. And the unglazed surface of the bisque area is completely smooth to touch and visual inspection.

These are slipcast porcelain mugs, fired to manufacturer' spec. They're not bloating, as they do not occur in any other areas of the mug.

Does this look familiar to anyone?

[JohnnyK]

Hey, moh...could you reshoot the photos with the focus on the bottom of the mugs instead of the rims? It would give us a better look at what seems to be crud in the bottoms, possibly from dust or dirt or even kiln wash flakes from the bottom sides of shelves placed above the mugs. Are the glazes yours or commercial? Is there some commonality between the white and the red glazes?

[neilestrick]

You're doing the casting yourself, or buying them pre-cast? Since they're in the bottom, I'd say it's something from the casting slip that's settling in the bottom of the mold during the casting process.

[Chilly]

1 hour ago, neilestrick said:

You're doing the casting yourself, or buying them pre-cast? Since they're in the bottom, I'd say it's something from the casting slip that's settling in the bottom of the mold during the casting process.

Agree with Neil.

Was it noticeable before bisque firing?

 

[Min]

When pouring the slip into the molds do you run it through a coarse sieve? What's your process for casting?

[Babs]

What is your glaze recipe?

One glaze I had dumped lumpy and rough  stuff onbottom of mugs

[moh]

@Min Yes, I run it through 60 mesh.

@neilestrick It's commercially bought. Visually there's nothing lumpy there to touch and visual inspection both greenware and bisque state.

The same glaze doesn't yield the same bumps on thrown mugs, so something's going on with the body. It's as silky smooth as a bisque can me.

The mug is very deep though, maybe that's causing issues? I'm trying a SLOW ramping now to see if that helps.

[liambesaw]

To me it looks like glaze pooling and boiling.  It happens when I pour and let the glaze stand in there too long, or if I am in a hurry and flip the mug or bowl over before the glaze dries and it starts pooling in the bottom.  Mostly happens in large bowls or tall cylinders.

[neilestrick]

Try to break them open, see if they're lumps or bubbles.

[Mark C.]

You have to bust them open to see what they are.Neil is on it

[moh]

I did end up grinding the footer off, then cutting it up where the bumps were. Didn't yield much in terms of how to fix it, but I did sense it's related to the slip body. 
I switched batches (it's a commercial slip) and the issue's fixed itself. Go figure.. A rough lesson when the order was for 200 mugs. (sobbing quietly)

[Babs]

Maybe worth takijng up witglh the source of the slip!