Bump in Bottom of Casting?

[cemoore]

I'm new to slip casting and want to understand an issue I'm having - hoping its an easy fix. I made some one piece shallow bowls  - finished size estimated at 5" diameter x 1" height, soft rounded bottom profile. Everything seems to working great with my commercial stoneware casting slip but I'm noticing raised areas/bumps at inside bottom of casting that look like they might be coming from where slip first hits the mold. I'm trying to move my pour stream around the bottom to avoid pouring all in one place, but still noticing. I read something about pouring out too fast could be creating a glug (not sure if this is slang or a term of art?) where mold might pull off plaster prematurely. I definitely have been pouring out fast. I am not noticing any imperfections on outside of casting (that was against plaster surface), rather just the inside, built-up wall. I don't have a picture at moment because I sanded them out.

Once I've poured out (currently at about 25 minutes time to get a bit over 1/8" wall), keeping item at about a 30 degree angle, I prop it up for about 10 minutes to where there doesn't seem to be a surface shine. Then I turn the mold right-side up. After roughly 1 hour I trim the sprue/excess material off the top rim using the trim ring and a knife, then waiting another hour or so before the item wants to release from the mold, which it does cleanly. I'm using a 3d printed raised plate to put down into the trim ring area so that my piece doesn't fall out and deform the edge or other parts. Then I sandwich the casting with another plate on bottom to turn it over to right-side up and gently push onto on a metal drying rack. I'm using some cloth rather than plastic over the casting for draft-less drying, cleaning up the rim and overall surfaces with a scotch-brite pad once bone dry.

Any tips for this newbie would be most welcome!

[High Bridge Pottery]

Have you tried sieving your slip? 

[Jeff Longtin]

I've read the post a few times and I'm still confused.

Long story short...you have a slight ridge in the bottom of your cast piece and it bothers you?  Have you tried using a metal scraper to remove the ridge after the clay has set up a bit?

It's not something I've really been aware of these days but I do think its an unavoidable part of the process. If you can start the pour in a part of the form where the bump is less noticeable that would make it less effort for you.

The other option, which I've employed on some forms, is to reverse cast the form. Have the mold form the INSIDE of the piece. A bit more challenging but it removes the ridge issue.

 

[Chilly]

Can you show us a photo please?

 

[PeterH]

> I read something about pouring out too fast could be creating a glug (not sure if this is slang or a term of art?) where mold might pull off plaster prematurely.

Glugging shouldn't be an issue casting bowls.

Potters use it with its normal meaning "a hollow gurgling sound or sounds as of liquid being poured from a bottle", and it is caused by the formation of a temporary airlock while filling or emptying the mould. This results in a partial vacuum inside the mould, and is usually only an issue with narrow-necked moulds.

A movie is worth a thousand words ...

 

A difficult mould to empty:

 

Edited July 8, 2023 by PeterH

[Jeff Longtin]

As best as I can tell here's what's happening in this situation...

when the liquid slip first makes contact with the dry plaster surface it IMMEDIATELY dries. Then, as more slip continues to flow over it, it forms a "ridge". 

In theory you have two solutions: as I mentioned previously, you can make the point of contact an area not visible from the top OR you can reduce the degree, to which the plaster dries the slip, at that first point of contact. You would do this by wetting the mold before you start casting. (Simply hold the mold, under running water, for just a second.)

Wetting the mold may reduce the height of the bump but it will also slow the casting process. Therein you have a choice.

Porcelain doll artists first introduced me to the idea of wetting a mold before casting. (To reduce sticking.) Subsequently I found it has other benefits as well.