One important issue to take in consideration, while dealing with slip casting, is that of the concentration of the slip, as it was discussed elsewhere at this forum: https://www.researchgate.net/post/What_is_a_typical_range_of_wt_solid_fraction_needed_to_obtain_a_concentrated_ceramic_slurry2

For a derivation of formulas relating slurry density with concentration, including Brongniart’s formula, you may want to check: https://www.researchgate.net/post/Can_anyone_suggest_a_smart_way_to_calculate_volume_fraction_of_a_solid_ferrofluid_in_a_solvent_or_solids_fraction

Also important is the particle size distribution, as pore clogging should be avoided for the benefit of the capillarity effect. Monomodal slurries are often reasonably well described by a simple log-normal distribution, as discussed at: https://www.researchgate.net/post/What_is_the_origin_of_log-normal_distribution_in_emulsion_droplets_and_other_particle_systems

As you are concerned with the slip casting of a silica slurry, you may possibly find some useful clues by checking the investigation entitled 'Glass art by slip casting and sintering recycled container glass', presented at the 'IV Portuguese-Spanish Congress on Ceramics and Glasses', Aveiro, Portugal, November 16-18, 2011; cf. 'Livro de Resumos' (Book of Abstracts), p. 129; and Poster:

Data Poster, 'Glass art by slip casting and sintering recycled co...

Conference Paper Glass art by slip casting and sintering recycled container glass

For possible useful clues to address the (problematic) shrinkage of a silica-based non-plastic green body, you may also want to check another discussion held at this forum:  https://www.researchgate.net/post/How_to_slip_casting_non_plastic_material 

thank you very much, you are such a helpful person Dr. Carlos Araújo Queiroz.

but my real question is the control  of sucking water with the plaster mold. we realize that defects are caused by the gypsum, not by the ceramic slurry. Thanks a lot!

The defects reason results from faster water loss. You can add  appropriate amount of water retaining agent, such as CMC, HPMC, PVA etc.

Dear Mirko Granese:

According to my experiences in slip casting, I can say that the defects that you saw are directly depends on the slurry rather than the mold used. Could you tell the particle size of the silica.  Did you used any binder, dispersant or slurry modifier during slurry preparation?

Also could you send some photos of the craks.

Best Regards.

If you are sure that the problem depends on the mold. Take a piece of wet sponge and apply the sponge to the inner surface of the mold just before casting. You will see the sucking speed of the mold will be reduce.

Best Regards.

Thanks Dr. Cui Xue-mi, We will test some of them, but I don't understand whether we have to add in the plaster mold or in the ceramic slurry.

Thanks Dr. Cem Özgür we don't use any kind of binder, dispersant or slurry modifier during slurry preparation (is only a trimodal silica particle size distribution, that resulta from a privius academic study ) . Can you suggest me someone?

If you are sure that the problem depends on the mold you should control/adjust the mold porosity (and this means the control the sucking water from the ceramic slurry used). For this kind of control you should study and adjust the gypsum /water ratio. Normally lower ratios (ex. 0.8; 0.9) we have strong porosity and fast water absortion but lower mechanical resistance of the molds. Highest ratios (more than 1.0) we have fine and lower porosities with strong mechanical resistance (higher durability). So, you should find the correct ratio for your case.

Only to give you an idea, normally in the slip casting of clay slurry, with 1.72 of density and 400 cp of viscosity, I use 1.0-1.2 of gypsum /water ratio.

Dr. Manuel Ribeiro I read this on a book "Principles of ceramics processing" (James S. Reed) and our next experiments will go in this direction. Thank you very much, now I'm more sure about the decision. Have you other titles of book about this kind of technology?

Dear Mirko Granese, in fact this is not a main subject of a specific book, but you can find some explanation/description on some traditional ceramic technological handbooks. Try this on: “An Introduction to the Technological of Pottery”, Paul Rado, (1990), 4th chapter.

Dr. Ribeiro thank you again, I should buy this book.

Dear Mirko Granese, In my experience the problems are usually with the slip and not with the mould. If you use a plaster that is manufactured for slip casting and have the right proportion between water and plaster there should not be any problems with the moulds.You can destroy a mold by drying it at to high temperature but apart from that the molds are usually very resilient. The design of the mold (geometric shape of your component) can also be very important for a good result. 

The most common cause of cracks are too rapid drying of the cast component. Fine powders (well below one micron) are more prone to  cause cracks during drying. A slip that is not properly deagglomerated (usually by milling) will tend to crack during casting. It is also very important to have a good collidal dispersion of the powder (use a dispersant adapted to the powder). This makes it possible to use a high volume concentration of powder in the slip and this makes cracking less likely. The best way to ensure that you have a good dispersion is to measure viscosity of your slip. Entraped air in the slip forms bubbles that cause pores in the cast component. Air bubbles have to be removed by slow stirring for several hours. It is also a good idea to pass the slip through a screen to remove any agglomerates or other contamination prior to casting.

These are some general advice on slip casting.

Thanks dear Prof. Elis Carlström, but the biggest problem in this moment is the formation of recess of material on the surface of artifacts. I think that the mold is sucking too quickly the water of the slip, this phenomenon obstacles the complete filling of the cavity. Can you suggest any kind of dispersant for a silica slip (we use a secret recipe of different distribution of only silica powder)?

Thanks a lot for the help!

Mirko Granese, concerning your last question I suggest you a specific book with the title "Ceramic Faults and their remedies" wrote by Harry Fraser.

wow thnks a lot Dr Ribeiro, my company ask to buy some book: there is a big help! Have a nice day!

Seem like you need a higher volume concentration of your slip. The casting rate is  very dependant on powder volume concentration. A good dispersant make it possible to increase this concentration and slow the casting rate. The cast layer will also pack better with a higher better dispersion and this also slows down the casting rate. Silica has a very low isoelectric point. For this type of materials you can often use polyethylene imine (PEI) as a dispersant. I have used PEI with SiC that also has a low isolectric point. Another trick is to wet the mould prior to casting to reduce the casting rate. Another trick is to  fill the mold with a solution of ammonium algiante prior to casting your ceramic suspension. This will cast a thin layer of alginate on the surface of the mold that will make separation of your component from the mold easier. 

Elis Carlström, thankls a lot.

Your suggestions will certainly be very useful. In this moment we already bathe the mold before the slip casting, but it is not sufficient to have a perfect control. Can you suggest me a scientific useful book for slip casting or for treatment/handling of slurries?

Best regards

The standard textbook would be: Principles of Ceramics Processing, 2nd Edition

James S. Reed, Wiley It covers the complete area of ceramics processing. If you want to know more about the fundamentals of dispersion and slip casting I would suggest Surface and Colloid Chemistry in Advanced Ceramics Processing Robert J. Pugh, Lennart Bergstrom, CRC Press

Best regards