Jeff Longtin

A simple way to find out if its your water...go into the bathroom and lift the lid on the toilet tank. More than likely the tank has a ring of some color. (Brown means you have iron in your water and blue green means you have something else?) 
If the tank water ring matches your butter dish ring I would presume its just a water issue. (Minerals in the water in other words.)

I'll put in a second for the Fred Olsen Kiln Book. His little Fast Fire kiln is easy to build and easy to fire. Plus, it's not too big, 10 cu ft. I helped build one years ago at the MN Ren Festival. Two fire boxes are a little tedious but it made it super easy to control.  Usually got to cone 7 just as the Festival was closing. (Once people were gone we'd throw in salt.) 
 

A simple way to find out if its your water...go into the bathroom and lift the lid on the toilet tank. More than likely the tank has a ring of some color. (Brown means you have iron in your water and blue green means you have something else?) 
If the tank water ring matches your butter dish ring I would presume its just a water issue. (Minerals in the water in other words.)

You said (I just didn't know its actually quantifiable.)
I think its to small to even consider

That looks really nice! What was the name of the paint you used?

To update: I mentioned my interest to an artist friend and he explained that there are two types of gold leaf: the real thing, expensive, and the imitation version, not so expensive. He also explained that applying gold leaf is very difficult and he would not recommend it. He said the better paints do a good job of creating a similar effect.
Turns out Amaco makes a non fire gold leaf paint. Maybe not as durable as gold lustre but alot easier on the lungs and studio.

Sort of, but ...
- The SG of water varies with temperature.
- Dissolved solids will increase the SG (mainly thinking of hard water).
Size of the effects on SG seem small enough to ignore "for government work"

https://www.hillcrestplumbingandheating.com/b/hard-water-vs-soft-water-debate-finally-solved
The Guidelines for Canadian Drinking Water Quality Hardness rank hard to soft by CaCO3 levels. Here are their guidelines:
Soft Less than 60 mg/L Medium Hard 60 to 119 mg/L Hard 120 to 179 mg/L Very Hard 180 mg/L or greater  

To update: I mentioned my interest to an artist friend and he explained that there are two types of gold leaf: the real thing, expensive, and the imitation version, not so expensive. He also explained that applying gold leaf is very difficult and he would not recommend it. He said the better paints do a good job of creating a similar effect.
Turns out Amaco makes a non fire gold leaf paint. Maybe not as durable as gold lustre but alot easier on the lungs and studio.

This page has a decent table for density variation by temperature:
Water Density | U.S. Geological Survey (usgs.gov)
Other variables to consider might include dissolved material, thermal expansion/contraction of measuring device?
I'm thinking there's another major variable, but can't put my finger on it just now...

One question jumps to mind: are you using the proper amount of water or "as little as possible"? Most casting plasters, I think, use 35-40% water. 
Do you have access to "pottery plasters"? Casting plasters, from my knowledge, have hardeners which make them more durable than pottery plasters but also less absorbent. 
If my memory serves me correctly there are three kinds of "plaster"; pottery plaster - for casting ceramic objects, gypsum cement - harder than pottery plaster but still absorbent, for machine pressing of ceramic objects, and casting plaster - designed for the giftware industry for very hard plaster objects which take paint well. Does your source differentiate between the three?
Clay - without deflocculant you need to add more water than is necessary. Have you tried using a deflocculant? In theory your clay has no elements that are antagonistic to the molds but it could be your clay does have things in it that are not friendly to your plaster molds. Those unknowns could be causing early deterioration? 

My class took place yesterday. The women and a few children had a fine time! 
Gave each a lumo of clay, got them to form into a ball, then hands under the table to shape for a few minutes, then pass to nneighbour, no peeking, x4 so clay now back with original owner, reveal all, keep, or squash.
Lits of laughs, then slab building, or pinching ,  or any of the above, coliured slips, or not..
I had a big bit of paper and they gave their name, drew a sketch of their pots and what glaze they would like listed.
Whew, did they have fun. Exhausted Babs trundled home.
Now to return topack and collect the pots, and home to dry , bisque and glaze. What gutsy folk are out there

@Jeff Longtin                            Wow, didn't think of making slabs for them.!! 
I have a lot of straight preserving "bottles".
I find beginners, unless taught properly, hmm, have quite a lot of failure re coils not squushed, scored and slipped.
Thickish slabs would give the choice of shaping,or/ and texturing.
Thank you Jeff.

Truth be told Kakes you are exactly a "typical ceramic artist".  No need to apologize. The major ceramic companies, Duncan and Amaco, designed their product lines to serve customers just like you. The ceramic choices at 06-04 are much more diverse than the choices at cone 6/10.
I work with a number of young people, recent ceramic grads, who are not concerned with durability or process but instead with simply making "interesting/unique" ceramic objects. I applaud their ambition as I applaud yours. 
Your results may not be, exactly what you want them to be, but if you lower your expectations a notch you will find some amazing results nonetheless.

Truth be told Kakes you are exactly a "typical ceramic artist".  No need to apologize. The major ceramic companies, Duncan and Amaco, designed their product lines to serve customers just like you. The ceramic choices at 06-04 are much more diverse than the choices at cone 6/10.
I work with a number of young people, recent ceramic grads, who are not concerned with durability or process but instead with simply making "interesting/unique" ceramic objects. I applaud their ambition as I applaud yours. 
Your results may not be, exactly what you want them to be, but if you lower your expectations a notch you will find some amazing results nonetheless.

Sweet pics. My studio is on the third floor of a six story warehouse. While I don't have pets, myself, several neighbors do. Little Bella, a Weiner dog, will stop in and say hello. The other neighbors like to hold her in their laps, when she visits, but I don't, so she sometimes gives me a disapproving look. Leo, a medium size mutt, likes to see if there is anything interesting in my garbage can. He likes to run in, check the garbage, and run out. He greets visitors to the floor with occasional barking. (Bella likes to bark, in general, if she's not getting enough attention from anyone.)
Pre covid we would have an open studio day, in April, where we would allow/encourage visitors to bring their pets in. As the stairwell doors would be open you would hear occasional barking all day.  It made for quite a social event, with dogs barking, and owners introducing themselves and their pets. It also made for good sales. Dog owners would take their dogs home and then come back later to purchase artworks.

Sweet pics. My studio is on the third floor of a six story warehouse. While I don't have pets, myself, several neighbors do. Little Bella, a Weiner dog, will stop in and say hello. The other neighbors like to hold her in their laps, when she visits, but I don't, so she sometimes gives me a disapproving look. Leo, a medium size mutt, likes to see if there is anything interesting in my garbage can. He likes to run in, check the garbage, and run out. He greets visitors to the floor with occasional barking. (Bella likes to bark, in general, if she's not getting enough attention from anyone.)
Pre covid we would have an open studio day, in April, where we would allow/encourage visitors to bring their pets in. As the stairwell doors would be open you would hear occasional barking all day.  It made for quite a social event, with dogs barking, and owners introducing themselves and their pets. It also made for good sales. Dog owners would take their dogs home and then come back later to purchase artworks.

Hey Kakes,
The first thing to mention is your terminology.  Typically stoneware clay is not considered "low-fire". It used to be considered "high-fire" (fired to cone 10) but more recently many bodies have been mixed to vitrify at a "mid-fire" temperature. (cone 6) 
Earthenware clay is more typically considered "low-fire". ( fired to cone 06) The two local companies, Minnesota Clay and Continental Clay, both sell earthenware clay but I know several potters who like to fire their earthenware pots to cone 2.
In a sense any vitrified clay is highly durable. In my experience, however, earthenware tends to be a bit more brittle than stoneware or porcelain clay, after firing.
As to which is best, that, I think, would be more about color range you want to work with. If you're finishing the pieces in glaze earthenware temps give you brighter colors and a broader color range.
Hope this helps?
 

Hey Kakes,
The first thing to mention is your terminology.  Typically stoneware clay is not considered "low-fire". It used to be considered "high-fire" (fired to cone 10) but more recently many bodies have been mixed to vitrify at a "mid-fire" temperature. (cone 6) 
Earthenware clay is more typically considered "low-fire". ( fired to cone 06) The two local companies, Minnesota Clay and Continental Clay, both sell earthenware clay but I know several potters who like to fire their earthenware pots to cone 2.
In a sense any vitrified clay is highly durable. In my experience, however, earthenware tends to be a bit more brittle than stoneware or porcelain clay, after firing.
As to which is best, that, I think, would be more about color range you want to work with. If you're finishing the pieces in glaze earthenware temps give you brighter colors and a broader color range.
Hope this helps?
 

Back in the 90"s I was commissioned to make 200 porcelain figurines. I found a sculptor to sculpt the piece and then I made 10 production molds. After showing the customer a cast piece, unfired, they asked me if I could paint the piece. As I knew they couldn't finish the piece I agreed to include that in the project. (It was for a group home for people with severe disabilities.)
Never having worked with "Amaco Underglazes" before I went out and bought about 20. Almost all of them were disappointing. (Except Medium Blue and Bright Yellow.) Frustrated I decided to "make my own".  I bought a pint of the Amaco LUG white Underglaze and a bunch of Mason Stains.
I experimented with mixing them, in various amounts, and found the homemade version to be quite good. There was a lot of frustration, but after awhile, I found a group of colors that safely went to cone 6 and were mostly opaque. 

That's really amazing Jes! 
Another option would be to make a mold of the piece. Use alginate. Apply a small amount to a corner and see how it responds. My guess is that it releases easily. (Perhaps a light spray of water to moisten the surface?)
After you've applied the alginate you would then back it with a plaster "mother" mold.
A bit time consuming but an easy way to capture such fine work.

Mason stains can be incorporated, but with caveats.
Stain particles are larger than ball clay ones, so you have to add stain to Sig after it’s been siphoned.  If you add stain before, most of it winds up in the discard sludge. Be aware you will likely have to mix frequently during application to keep everything evenly suspended. But it does give nice results. 
Also, if you use any sig methods involving ball milling, that can alter several stain colours. It isn’t recommended for encapsulated ones at all.
 

That sounds like fun Babs. Congrats!
I would steer the women towards coil building mostly. That allows them to make a pot, of any size, of their choosing. Any time I tried to pinch a pot I was always limited to making small cup forms. (To make a tall pinch pot you really need to let the clay stiffen for a period of time. Is that something you can do in this limited situation?)
If you choose to offer slabs of clay maybe you could bring coffee cans or plastic bowl forms that they can use to mold the slabs? Maybe cardboard tubes as well?
When I was in my 20's I worked part time at a daycare center down the street from my studio. One day the head teacher, Dawn, asked me to introduce the kids to clay. For each of the kids, 15 in total, I made small slabs of clay and a few coils. Seven of the children decided to make nests with their slab and coils. What was really cute was that each off the kids had a little story to tell about their nest. (Each had also made little ball forms which represented the eggs.)
The whole experience went in a direction I did not expect and was rewarding as a result. I hope you have similar success.

That sounds like fun Babs. Congrats!
I would steer the women towards coil building mostly. That allows them to make a pot, of any size, of their choosing. Any time I tried to pinch a pot I was always limited to making small cup forms. (To make a tall pinch pot you really need to let the clay stiffen for a period of time. Is that something you can do in this limited situation?)
If you choose to offer slabs of clay maybe you could bring coffee cans or plastic bowl forms that they can use to mold the slabs? Maybe cardboard tubes as well?
When I was in my 20's I worked part time at a daycare center down the street from my studio. One day the head teacher, Dawn, asked me to introduce the kids to clay. For each of the kids, 15 in total, I made small slabs of clay and a few coils. Seven of the children decided to make nests with their slab and coils. What was really cute was that each off the kids had a little story to tell about their nest. (Each had also made little ball forms which represented the eggs.)
The whole experience went in a direction I did not expect and was rewarding as a result. I hope you have similar success.

Thank you for the comments - i will try as suggested, a much better way!
 

I think Roberta has the better suggestion. The depth of the fabric makes it very challenging to plaster. Rather than have that challenge just roll the fabric into a soft clay slab first and see if you like that.