Hyn Patty

I've refired old and dirty bisques many times before.  Clients send me pieces from all over the world that are decades old and been handled with oily hands, etc.  I refire these clean as a matter of standard procedure before I attempt any glazing so that I know the bisque is super clean before I proceed.  I agree, wash off what you can but if the clay may be porous at all, do NOT SOAK in water.  Water can dissolve salts and calcium out of your clay so merely wash up with a scrubby and a little soap, then rinse.  Set aside to dry.  I use a heater fan to help such pieces dry faster.
If you don't want to wait days to be sure the bisque is dry enough to fire, I go ahead and load it up into the kiln and fire on a low temp pre-soak for a couple of hours  before going ahead with the new bisque firing.   Cooks off any moisture left but do it slowly.  Obviously that is not an issue if the ware is fully vitrified  but I advise it for porous earthenware.
If after firing you are left with any dirty grey ghost marks you may sand the bisque to remove it if the glaze or underglaze you plan to apply isn't opaque enough to conceal it.  Alternatively you can also add a layer of opaque white underglaze and fire that before you proceed with color.

I've refired old and dirty bisques many times before.  Clients send me pieces from all over the world that are decades old and been handled with oily hands, etc.  I refire these clean as a matter of standard procedure before I attempt any glazing so that I know the bisque is super clean before I proceed.  I agree, wash off what you can but if the clay may be porous at all, do NOT SOAK in water.  Water can dissolve salts and calcium out of your clay so merely wash up with a scrubby and a little soap, then rinse.  Set aside to dry.  I use a heater fan to help such pieces dry faster.
If you don't want to wait days to be sure the bisque is dry enough to fire, I go ahead and load it up into the kiln and fire on a low temp pre-soak for a couple of hours  before going ahead with the new bisque firing.   Cooks off any moisture left but do it slowly.  Obviously that is not an issue if the ware is fully vitrified  but I advise it for porous earthenware.
If after firing you are left with any dirty grey ghost marks you may sand the bisque to remove it if the glaze or underglaze you plan to apply isn't opaque enough to conceal it.  Alternatively you can also add a layer of opaque white underglaze and fire that before you proceed with color.

I've refired old and dirty bisques many times before.  Clients send me pieces from all over the world that are decades old and been handled with oily hands, etc.  I refire these clean as a matter of standard procedure before I attempt any glazing so that I know the bisque is super clean before I proceed.  I agree, wash off what you can but if the clay may be porous at all, do NOT SOAK in water.  Water can dissolve salts and calcium out of your clay so merely wash up with a scrubby and a little soap, then rinse.  Set aside to dry.  I use a heater fan to help such pieces dry faster.
If you don't want to wait days to be sure the bisque is dry enough to fire, I go ahead and load it up into the kiln and fire on a low temp pre-soak for a couple of hours  before going ahead with the new bisque firing.   Cooks off any moisture left but do it slowly.  Obviously that is not an issue if the ware is fully vitrified  but I advise it for porous earthenware.
If after firing you are left with any dirty grey ghost marks you may sand the bisque to remove it if the glaze or underglaze you plan to apply isn't opaque enough to conceal it.  Alternatively you can also add a layer of opaque white underglaze and fire that before you proceed with color.

I've refired old and dirty bisques many times before.  Clients send me pieces from all over the world that are decades old and been handled with oily hands, etc.  I refire these clean as a matter of standard procedure before I attempt any glazing so that I know the bisque is super clean before I proceed.  I agree, wash off what you can but if the clay may be porous at all, do NOT SOAK in water.  Water can dissolve salts and calcium out of your clay so merely wash up with a scrubby and a little soap, then rinse.  Set aside to dry.  I use a heater fan to help such pieces dry faster.
If you don't want to wait days to be sure the bisque is dry enough to fire, I go ahead and load it up into the kiln and fire on a low temp pre-soak for a couple of hours  before going ahead with the new bisque firing.   Cooks off any moisture left but do it slowly.  Obviously that is not an issue if the ware is fully vitrified  but I advise it for porous earthenware.
If after firing you are left with any dirty grey ghost marks you may sand the bisque to remove it if the glaze or underglaze you plan to apply isn't opaque enough to conceal it.  Alternatively you can also add a layer of opaque white underglaze and fire that before you proceed with color.

This is totally not my kind of work but WHAT A COOL THREAD!  Thank you to everyone who added input, details about how to do this, photo examples.  You are all the best.  I may have to try this some day - as if I don't already have more on my plate than I have time to keep up with!  Hahaha.  But, nice read.  Thanks for sharing.

This is totally not my kind of work but WHAT A COOL THREAD!  Thank you to everyone who added input, details about how to do this, photo examples.  You are all the best.  I may have to try this some day - as if I don't already have more on my plate than I have time to keep up with!  Hahaha.  But, nice read.  Thanks for sharing.

There are so many articles available on this subject: OSHA, CDC, NIOSH, your public health and government sites. All can provide good guidance so you should read for your state for sure at a minimum. HEPA filtration is the only recognized filter that I know approved for effective removal especially in air filters and vacuums. Minimizing generation is super important so house keeping, wet not dry methods etc….. are most important.
Personal protective equipment such as N95 rated masks or other rated masks are also a common recommendation when sanding, glaze prep, etc…. I would start with google and your local health department. Avoiding generating dust is usually a very good first practice which consists of: don’t generate it indoors, so wet cleanup, hepa vacuum, etc… are common. 
Some things you may see with clay and most clay studios
Vacuum your kiln after firing : use a real hepa rated vacuum. Air filter: hepa rated Mix glazes: do this outdoors with N95 mask or superior for personal protection or in an approved paint spray booth, or with approved exhaust. Cleanup: wet cleanup only, no brooms or brushes etc… Read through the documents, much has been written with most requirements unified under OSHA for employers / Employees in the US and uniform exposure standards through federal and state health standards. Your local health department should have plenty of information available right on their website.
All else fails, NIOSH has publicized suggestions as well for construction practices and personal filtration and protection. You may find them useful. Your source of silica are basically clays and glazes whereas construction activities pertain to cementitious mixing, grinding etc….. https://www.cdc.gov/niosh/docs/96-112/default.html#:~:text=Use vacuums with high-efficiency,being released in the air.. Your state health department likely has written materials that may be easier to read through, the NIOSH is just one of many.
It does not matter which source ………..  how to minimize, how to protect and how to collect are the relevant issues.

I don't know if this was already mentioned, but I just saw a video of someone who made their own screens using a store bought screen, but instead of using emulsion she used her Cricut machine to cut peel-and-stick vinyl. She then adhered it to the screen and it functioned like emulsion would. She was screening on fabric, but I don't see whey it wouldn't work to screen onto tissue paper to make your own stencils. She could then peel the vinyl and re-use the screen for another design. I'm sure there are limitations to this method, but if you have a Cricut it seems like a good option. I'm going to give it a try if I can find the time.

There abouts, yes.  Cone 016 to 018 range would be fine. 
This is NOT something that traditional china painting groups know about or use.  Trust me, I've been pursuing this for a  while.  I've chatted with countless suppliers and top workshop teachers.   Everyone either wants high gloss or at most uses matting agents to achieve satin finishes by adding it directly to the pigments.  That's the standard.  There is only one company I have found that still bothers with matte china paints at all.  It's long since fallen out of fashion, to be honest.  I did just pick up a /matte/ additive I am testing (rather than satin) but it also had no effect when added to a clear gloss flux.
So what I am looking for here is something very unusual among over glaziers but isn't absolutely /unheard/ of.  So I'll check the group linked to but yeah that's not likely a helpful source.  I am the queen of doing things outside of the norm or that people insist simply can't be done - and then making breakthroughs and finding ways to make it happen anyway!  It's kind of my credo.    If it's impossible, I accept the challenge and make it happen.
The other thing I'm looking for are NON TOXIC over glazes that are OPAQUE.  The enamels I have and use currently for that purpose are full of lead and cadmium and I just HATE to put that stuff through an airbrush - no matter what health precautions I take.  They are however compatible with china paints and fired in the same ranges more or less.   It allows me an option to go back and lighten pieces in select areas that just get too dark.  I do have a few leads there that I'm looking into but I haven't nailed anything down yet in the firing range I require.

There abouts, yes.  Cone 016 to 018 range would be fine. 
This is NOT something that traditional china painting groups know about or use.  Trust me, I've been pursuing this for a  while.  I've chatted with countless suppliers and top workshop teachers.   Everyone either wants high gloss or at most uses matting agents to achieve satin finishes by adding it directly to the pigments.  That's the standard.  There is only one company I have found that still bothers with matte china paints at all.  It's long since fallen out of fashion, to be honest.  I did just pick up a /matte/ additive I am testing (rather than satin) but it also had no effect when added to a clear gloss flux.
So what I am looking for here is something very unusual among over glaziers but isn't absolutely /unheard/ of.  So I'll check the group linked to but yeah that's not likely a helpful source.  I am the queen of doing things outside of the norm or that people insist simply can't be done - and then making breakthroughs and finding ways to make it happen anyway!  It's kind of my credo.    If it's impossible, I accept the challenge and make it happen.
The other thing I'm looking for are NON TOXIC over glazes that are OPAQUE.  The enamels I have and use currently for that purpose are full of lead and cadmium and I just HATE to put that stuff through an airbrush - no matter what health precautions I take.  They are however compatible with china paints and fired in the same ranges more or less.   It allows me an option to go back and lighten pieces in select areas that just get too dark.  I do have a few leads there that I'm looking into but I haven't nailed anything down yet in the firing range I require.

Mark makes a good point. Glue is for broken molds, two pieces, not "cracked".
Cracks, on the inside of the mold, can sometimes be filled with a thin slurry of plaster, but sometimes not. It's usually best to apply the plaster in a water like consistency and build it up in layers. This repaired crack will likely be a spot of sticking, for the cast piece, but after several castings the sticking should decrease.
 
 

For those of you who do overglazing using china paints (or enamels for that matter - I use both as well as underglazing) flux is available to add to pigments to make them shinier or to use as a final top coat. 
There are various materials like zinc oxide that can be added as a matting agent to dim the gloss of overglazes.  This is normally added directly to the pigments to make them satin or velvety matte in the final finish.  Using too much though will lighten the colors, making black for example turn into a dark grey.
 BUT there doesn't seem to be an option for making a final top coat for a satin or matte finish to make all of the overglaze uniform.  I've tried adding matting agents to clear gloss flux and my tests still come out high gloss.  I have no option obviously of reducing the silica content of the flux, only adding tons more of the matting agents.  I will try more testing and much higher ratios to see if I can possibly make a dent in the flux high gloss but I'm worried it will fog. 
If anyone here has any suggestions, I'd love to hear it!  I've searched and searched and made lots of phone calls to no avail from china painting dealers.   I have one lead I am chasing but more leads would be good.

Yes, I've used all of this before and notes are not what I wanted.  Yesterday in the 'Manage Image' drop down menu 'Edit Details' was not there!  I went back and tried it several times yesterday and it wasn't showing up or the link wouldn't work and load the next page to do so.  BUT it does seem to be working today.  I just logged in and tried it and it works fine now.  I don't know what the issue was but it was definitely driving me nuts because I have used that feature many times and it simply wasn't working this time.
Anyway, I don't know if someone fixed it, or if it was merely bad luck that that data was getting interrupted every time I tried it.  Thanks for looking into it either way!

I'll check with admin on this, stay tuned.

I'd go with one of the bisque fix materials myself.  Adding paper IMO just makes for a bulkier but weaker filler.  Once fired the paper will leave a lot of voids.  Great for light weight, not so much for strength.  If you can roughen up the edges of the cracks you'll get a stronger join so the bisque fix has something to 'grab' and hold together better than smooth surfaces.  Some small diamond coated files or diamond coated dremel tips will make short work of roughing and slightly widening the cracks.  Good to remove any loose pieces too that would otherwise be at risk of flaking off.

Just a heads up to anyone who might substitute Sierralite talc for Amtalc-C98, there is a significant difference in chemistries between the two talcs. A direct 1:1 substitute likely won't work unless it's just a very tiny amount of talc being used. Main differences are in the silica and alumina levels.

I use sierra lite from laguna-its a white talc-no stortage, cost is more.

+1 for Ozone removing ALL of the odor (100%). Reasonable precautions of course - it is an irritant. The good news it’s used for water purification, removing smoke smell from fires, cars with years of cigarette smoke etc…. So extremely useful.
More good news, It’s hard to make more than about 3-4% concentration of O3  and it reverts to O2 in a fairly short time. If you set this up outside, then a box or simple plastic garbage bag is ideal. You do not want the generator to recycle its own output so basically a way to blow ozone / fresh air and let it leak out naturally.
Humans can smell extremely low concentrations of Ozone which contributes to its relative safety, sort of like mercaptan in natural gas products, most people can smell it very quickly. So if you do this, do it in a well ventilated area (outdoors) and stay clear while operating. 10-15 minutes exposure is often enough to remove any trace of odor and 30 - minutes post ozone production is often enough for all the ozone to revert to oxygen on its own.  A decent ozone generator ought to have specific warnings so read and follow all precautions for sure. 

Thank you all for answers and tips. I switch the clay to more fine one and the issue went away. Now I have pretty good results. Thanks again

Good advice here already but I have a suggestion.  Swap out your stoneware embedding clay for oil clay. 

Thanks so much! I stumbled onto his videos a few days back and they are great but I think I will take your advice and just put the laptop down!! 

I'll pick up on what Hyn Patty says: some of us started making molds before the internet and we really only learned from our mistakes. In theory plaster is inert and should behave the way we expect it to.  Unfortunately that is not always the case and you need to improvise. That's when new ideas are born and new methods created. Continue your trials and feel free to post questions when they arise.
 

You already got great reponses here so I  have little to add!  Most of my originals are sculpted in oil clay and initially I did all of my plaster molds directly from them.   If you are careful to use oil clays that are sulfur free you can also avoid most issues with rubber conflicts.   As already noted however, if you are pouring large plaster sections the heat can cause your oil clay originals to melt and deform just like with wax originals.
Once you get some practice and confidence, and you move into higher end pieces you invested a lot of time into sculpting, you may also wish to consider casting rubber masters.  If you make a rubber master of your original, and rubber master pieces of your mold parts you planned to use in plaster, then you can simply recast more plaster production molds over and over again from your rubber masters.  Plaster molds, especially those with very fine detail such as my work, are only good for about ten to twenty casts before the molds will begin to show wear.  The more detailed and complicated the mold the faster you will see problems, especially if you do not clean and store your plaster molds properly after each use.  Simple, low detail molds will last a lot longer.
VanTiki posts wonderful Technical Tuesday Youtube videos that may be hugely helpful to you to learn mold making.  There are also some great blog posts out there if you do internet searches.  Here's a great video on rubber master molds for plaster slip casting right here.  He posts a lot of videos on regular plaster mold making so look him up!
 

Here is another VanTiki video, this one only focusing on making a plaster mold.  This one also shows you about how to make your mold walls using wood (usually a simple box you can slide to adjust sizes, this one is more of a chair form.  You can also make your molds using Lego blocks instead of sealed wood which is my choice when I'm making smaller molds to pour.
 
 

Just want to say BRAVO!  Have no fear and  jump right in.  Even if you ruin it, you'll learn from it.  Biggest problem I run into teaching is that people are /afraid/ of failure.  I've killed countless plants learning how to grow and propagate them, and I've ruined many a painting and sculpture, mold, and casting.  But I learn best by DOING.  So jump right in and don't be afraid.  You got this!  Start simple, take small steps and if you need help, that's what we are here for!