Slipcast cracking issues. Help!

[prattcm]

I have been slipcasting recently using premixed mayco stoneware slip. I mix it well before pouring but there are always some darker swirls that you can see at the top of the bucket. They are noticeable when I pour the pieces where the slip reaches the top of the mold and meets in the middle. I’ve had a few pieces that show minor surface cracks in this area. What is causing this and how can I prevent it?

[PeterH]

Nice picture of the swirls, but I'm having some difficulty visualising the flow in your mold. The picture looks like the swirl is on a vertical side of a cylindrical shape. While you say it's "where the slip reaches the top of the mold and meets in the middle" which suggests that it's horizontal when casting.

Perhaps a picture of the mold, or of the piece in it casting orientation would clarify things?

[prattcm]

7 minutes ago, PeterH said:

Nice picture of the swirls, but I'm having some difficulty visualising the flow in your mold. The picture looks like the swirl is on a vertical side of a cylindrical shape. While you say it's "where the slip reaches the top of the mold and meets in the middle" which suggests that it's horizontal when casting.

Perhaps a picture of the mold, or of the piece in it casting orientation would clarify things?

It is molded in this orientation and the swirls form up near the hole on the left and wrap down to that side

[PeterH]

Thanks. Do you have any sort of reservoir? Otherwise filling is going to be slow near the end, which may exacerbate any other problems with the slip.

[liambesaw]

What do you use as a deflocculant?  It looks like it may be over deflocculated a bit, to the point it's separating into layers

[prattcm]

54 minutes ago, PeterH said:

Thanks. Do you have any sort of reservoir? Otherwise filling is going to be slow near the end, which may exacerbate any other problems with the slip.

I have a 5 gallon bucket up on a shelf with a hose coming out of the bottom that I use. It comes out pretty slowly though. I can try a different way to fill it more quickly to see if that fixes the issue.

[prattcm]

56 minutes ago, liambesaw said:

What do you use as a deflocculant?  It looks like it may be over deflocculated a bit, to the point it's separating into layers

So I get this slip in 2 gallon bags premixed. I was wondering if this looks like there could be an issue with the mixture. I could try to talk to the supplier about it the next time I buy more. 

[liambesaw]

7 minutes ago, prattcm said:

So I get this slip in 2 gallon bags premixed. I was wondering if this looks like there could be an issue with the mixture. I could try to talk to the supplier about it the next time I buy more. 

Ahh ok, in that case I don't think there is a lot you can do to fix that issue.  Is there a reason you don't mix your own?  There are some great easy to mix porcelain slips

[prattcm]

22 minutes ago, liambesaw said:

Ahh ok, in that case I don't think there is a lot you can do to fix that issue.  Is there a reason you don't mix your own?  There are some great easy to mix porcelain slips

I haven’t been doing this whole ceramics thing too long, but I’ve just been using it because of the simplicity. I’m sure I will mix my own before too long, but haven’t taken the leap yet.

[liambesaw]

3 minutes ago, prattcm said:

I haven’t been doing this whole ceramics thing too long, but I’ve just been using it because of the simplicity. I’m sure I will mix my own before too long, but haven’t taken the leap yet.

Ahh ok!  Your stuff looks 3d printed or professionally molded so you probably need a specific shrinkage as well, so that might be tough if it's already sized for the commercial slip

[prattcm]

1 hour ago, liambesaw said:

Ahh ok!  Your stuff looks 3d printed or professionally molded so you probably need a specific shrinkage as well, so that might be tough if it's already sized for the commercial slip

Yeah I 3D printed the original model and made the mold myself. Picked a nice easy project  to start with... anyway it’s stoneware slip and I think the shrinkage is around 11% if I remember correctly. I’ve looked and I think I should be able to find some dry mix that is close enough.

[liambesaw]

Looks good. Will have you have you come over and show me how you make your molds because mine are always ugly.

[PeterH]

1 hour ago, prattcm said:

I have a 5 gallon bucket up on a shelf with a hose coming out of the bottom that I use. It comes out pretty slowly though. I can try a different way to fill it more quickly to see if that fixes the issue.

Unless you overfill (i.e. into a reservoir) I cannot see how you can add slip rapidly towards the end (or avoid the need for continual topping up). I don't know that this is a problem, but it feels all wrong. I would at least try with a clay or plasticine cottle so you can see the effect of filling the entire mold at full flow-rate.

[Mark C.]

Molds need a larger area at top of spur to fill extra so as the walls thicken it drops down but is above mold level.You can and may have to top this off as well as the level drops. Molds made without this reservoir will have issues.You can add this top with a collar to yours as it looks like this may be an issue

[prattcm]

Sorry for the confusion. This is what the mold looks like closed. I have two silicone funnels I put in each hole. The slip does not drop below the mold level during casting. These lines happen during the initial filling of the mold

[Min]

@prattcm, since you have a 3D printer you could print out a collar and use a coil of clay to attach it to the mold. 

edit: we posted at the same time, ignore what I wrote above. 

I'ld get the slip into the mold faster and start measuring the specific gravity if you aren't already doing that. It might need adjusting.

 

[Mark C.]

Mold collars will help

[PeterH]

3 hours ago, prattcm said:

Sorry for the confusion. This is what the mold looks like closed. I have two silicone funnels I put in each hole. The slip does not drop below the mold level during casting. These lines happen during the initial filling of the mold

@prattcmRight, are you just pouring through unattached funnels, or do they form an extension to the mold:

Obviously this forms a reservoir, but you don't want the speed of the pour over-restricted by a long narrow funnel neck. (Just realised I don't understand how the last of the air escapes if you have two long funnel necks dipping below the surface of the slip in the mold. Maybe it's better to have a filling funnel in the lower hole and a collar round the other hole.)

Looking at the top pix: using a slightly sloping mold must influence the manner in which the slip meets the top of the mold. This can help avoid air-pockets, but it may also help reduce wreathing. Anybody have any thoughts?

Agree with the suggestions that the slip may be over deflocculated, but using a shop-bought premix makes this difficult to address (you cannot remove deflocculant). As Min said, it's worth checking the specific gravity, you can at least reduce this if appropriate.

[PeterH]

Managed to find a DIY slip viscosity test, anybody got a better one (with target value(s))?

in https://aardvarkclay.com/pdf/technical/Basicsofslipcasting.pdf
Step2. VISCOSITY OR FLOW TEST
To determine the proper amount of deflocculant, you must measure the viscosity/flow of the slip. The viscosity/flow test measures the time for a set volume of slip to flow out of a container. A viscometer that measures slip flow over a period of a minute or longer is recommended. An alternative is to construct your own. Use a 10" piece of 2" diameter PVC pipe. Connect a cap with a 5/32" hole drilled in the center. Turn the container over so the hole is facing downward, place your finger over the hole, fill the container to the rim with slip, and then let the slip drain into a 250ml .graduated cylinder. Time the flow from when you remove your finger until the slip fills the graduated cylinder to the 250 mark. The time should be about 25 seconds for good slip

[prattcm]

3 hours ago, PeterH said:

@prattcmRight, are you just pouring through unattached funnels, or do they form an extension to the mold:

Obviously this forms a reservoir, but you don't want the speed of the pour over-restricted by a long narrow funnel neck. (Just realised I don't understand how the last of the air escapes if you have two long funnel necks dipping below the surface of the slip in the mold. Maybe it's better to have a filling funnel in the lower hole and a collar round the other hole.)

Looking at the top pix: using a slightly sloping mold must influence the manner in which the slip meets the top of the mold. This can help avoid air-pockets, but it may also help reduce wreathing. Anybody have any thoughts?

Agree with the suggestions that the slip may be over deflocculated, but using a shop-bought premix makes this difficult to address (you cannot remove deflocculant). As Min said, it's worth checking the specific gravity, you can at least reduce this if appropriate.

When I pour it is very similar to the picture you shared. The funnels I’m using fight tight and form an extension. They only go in the mold about 1/2 inch, so they don’t prevent the air from escaping. The hole in the bottom of the funnel matches the hole in the mold, which is rather large so is not limiting the flow into the mold. I will get a large pitcher so I can pour it more quickly and see if that solves it. Thanks!