Slip mixing tips

[2Relaxed]

I have a 50lb bag of Laguna Glacier casting slip. I'm not  interested in casting this porcelain (bought it for another experiment that I now abandoned). So I decided to try mixing it to use as engobe for the interior of pots made of dark clays. Mixed per instructions I found on the *Internet*: 50lb of dry mix per 3 Gal of water to get 5 Gal of slip, and add some Darvan to deflocculate.

Since I'm only testing it for now, I mixed a smaller batch: 10lb to 2.27L and added 4 drops of Darvan. The slip I got is very thick, the consistency of pudding almost. Not sure what the reason is. Might be my Darvan is too old? I think the jar I have is at least 2 years old. This consistency is definitely not suitable for pouring into a pot and then pouring out. 

What would you do? Add more water? Try to deflocculate further?

[liambesaw]

If it for engobe it should be fine nice And thick!  If you want to dip, add more darvan

[2Relaxed]

@liambesaw Really? I had a poor experience recently with a glaze that was almost the same consistency, I did not like how thick it went on - with streaks and globs. And of course, it failed in the glaze fire LOL 

[Min]

51 minutes ago, 2Relaxed said:

10lb to 2.27L and added 4 drops of Darvan

Is this Darvan 7 you are using? (if it's Darvan 811 then the math below will be off)

If you are trying to make a casting slip 4 drops of Darvan 7 isn't enough for 10 lbs of dry mix. To make a casting body Darvan 7 amount would be approx 0.35% of the weight of the dry materials, this would probably be the maximum amount you want to add, could very well need less, each casting body is going to vary slightly. Water amount is appox 35% of dry weight, again this has wiggle room depending on what's in the casting slip dry recipe.

So for the math - 35% of 10lbs (of dry mix)  for the water would be 3.5 lbs which I believe would be 1.59 litres.

For the darvan 7 -  0.35% would be 16.3 grams.

Add the darvan to the water before mixing it into the slip, don't use all the darvan or water at once, you might not need the full amount of either. There should be a specific gravity range the supplier gives for this casting slip. If when you weigh it out the slip is in the correct range but it is behaving like it's too thick then add more darvan. If the weight is too low then you need to add more dry mix first to get the specific gravity in the correct range then correct the darvan for flow if necessary. Casting slips seem to behave better if left to sit for 24 hours after mixing before using them, mixing them for a sufficient amount of time is beneficial too.

With my math you have added too much water so I would expect the specific gravity to be low. Also, I don't think you've added enough Darvan 7 so I would expect it to appear thick.

Other approach would be to not bother making it into a casting slip at all. Just add water until it's a the consistency you want and try it like that. I would probably try it both ways but just scale down the amount you try to say a pound of slip and a small cup of plain water/dry mix with no Darvan 7. 

[2Relaxed]

2 hours ago, Min said:

With my math you have added too much water so I would expect the specific gravity to be low. Also, I don't think you've added enough Darvan 7 so I would expect it to appear thick.

Other approach would be to not bother making it into a casting slip at all. Just add water until it's a the consistency you want and try it like that. I would probably try it both ways but just scale down the amount you try to say a pound of slip and a small cup of plain water/dry mix with no Darvan 7. 

Min, thank you so much. I used Darvan 7. 

So, since the 10lbs batch is already mixed, what should I do? Since there's so little Darvan in it, maybe I could ignore its presence entirely and just add more water to get the consistency that is usable? 

[Min]

Yes, that could very well work. I'ld just take  out a cup or so of slip and add a few drops of Darvan until it flows better and try that on test pieces. I'ld also try another cup of slip and add water until it's still thick then add a tiny bit of Darvan until it flows.Try a third cup of slip like you suggested, ignore the tiny bit of Darvan and just thin the slip out with water and test that also. Measure both specific gravities and test them. Might be fine using one with a lower specific gravity and no extra Darvan. When you test the slip check how it flows on the dark clay when you apply it as well as how it covers and if it fits.

[2Relaxed]

14 hours ago, Min said:

Yes, that could very well work. I'ld just take  out a cup or so of slip and add a few drops of Darvan until it flows better and try that on test pieces. I'ld also try another cup of slip and add water until it's still thick then add a tiny bit of Darvan until it flows.Try a third cup of slip like you suggested, ignore the tiny bit of Darvan and just thin the slip out with water and test that also. Measure both specific gravities and test them. Might be fine using one with a lower specific gravity and no extra Darvan. When you test the slip check how it flows on the dark clay when you apply it as well as how it covers and if it fits.

Sounds like a great plan and that's what I will do! Thank you again!