Author Topic: Plaster of Paris, Investment, etc.  (Read 19639 times)

Offline vtsteam

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Plaster of Paris, Investment, etc.
« on: August 03, 2014, 09:37:47 PM »
So I've been doing some online reading about lost wax casting, investments, homemade and commercial, and am coming across a lot of conflicting information.

First question I had was, what is the calcining temperature of plaster of Paris. You'd think that would be a pretty straightforward figure. Well it isn't. I've seen:

About 600C
About 650C
About 600 F

"It is obtained by heating gypsum or calcium sulfate dihydrate to about 140°C - 180°C. "

"It is necessary to take the investment to the calcining temp (~650°F) at a controlled rate of rise so as to maintain its integrity. Once it it fully calcined it can be raised to 1200°F fairly quickly."

"I would strongly suggest that you obtain some true casting investment rather than trying to use homemade stuff. Plaster of Paris cannot withstand the high temperatures (1200°F/650°C) needed to get a clean burnout."

"The book I use as my guide says that 48hours burnout at 600 deg C should be enough more than that is just overkill."


Okay, what the heck?

Is this like a bunch of blindfolded people feeling up an elephant for the first time and trying to describe what they imagine the animal is?

What is THE calcining temperature of Plaster of Paris? And please cite your reference.

Pretty please?
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline mattinker

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Re: Plaster of Paris, Investment, etc.
« Reply #1 on: August 04, 2014, 03:48:25 AM »
My source was the "Fondrie Godard", a Parisien foundry who now work for the musé Rodin amongst. My girl friend of some twenty years ago worked for the Stanford in Paris program and one of the things she organised annually was a visit to the Godard Foundry,so I got myself in! At the time, there wasn't all the conflicting material available on the Internet. It was hard to find concrete information as there is a lot of mystique around lost wax. I got the 600°C from the horses mouth! The founders that I met and spoke to about the process were adamant about soaking the mold in heat at 600°C to make sure that there was no water left. One of the differences between sculpture and mechanical parts is the thickness of the casting. Sculpture is usually made as thin as possible to keep the cost of material as low as possible, the 600°C preheat allows the molten metal to flow more easily. If the mold is left to cool, it will reabsorb humidity from the air.

"It is obtained by heating gypsum or calcium sulfate dihydrate to about 140°C - 180°C. " This applies to the transformation of Gypsum into plaster. The rock is heated and crushed to form plaster, this is an irreversible process, once it has been turned into plaster, it cannot be recycled back into reusable plaster.

I started working with plaster over  forty five years ago, first of all casting and running moldings, later here in Paris I did a lot of building work in Plaster. The home of plaster used a lot of plaster in places where cement is used else where. One can still buy "coarse" plaster which is used to set window frames and beams, ordinary plaster and dental or casting plaster. Investment plasere needs to be pure plaster with a grog added, brick dust or silica sand.

It is about twenty years since I did any lost wax casting, the preheating kiln being a big limitation. I have a project to build one in a 200 liter oil drum, I hope to get this done before I retire in three years time! I sill have sculpture in mind to cast!

Regards, Matthew

Offline tom osselton

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Re: Plaster of Paris, Investment, etc.
« Reply #2 on: August 04, 2014, 04:42:31 AM »
I have been surfing tonight and have found this site he did his research masters degree thesis on this subject. he talks about ash content, shrinkage and burnout times varying on the size of the casting.

http://www.calumdouglas.ch/metal-casting/

This next site is using abs plastic but states that it is under proper ventilation as it is toxic!

 http://jason-webb.info/2012/11/lost-abs-experiment-with-3d-printed-objects-and-aluminum-casting/#burnout

This is the utube video that I saw originaly he says he has 20 years experience casting metal and fires his plaster mold after a couple hours of setting and it seems to go fine, he says because of the sand content.



Heres a pdf for HYDROCAL®Gypsum Cements

http://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/hydrocal-gypsum-cements-application-en-IG538.pdf
 
« Last Edit: August 04, 2014, 05:17:12 AM by tom osselton »

Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #3 on: August 04, 2014, 07:07:22 AM »
Thanks Matt, great to hear about your sculpture casting experience in France. That must have been really interesting to be a part of.

And Tom thanks for the references that adds even more "takes" on investment casting. Obviously there is a huge range of opinion and procedure.

Now let me clarify my question, because I haven't seen in any of the above information the actual answer to it. Yes there is lots of information about investment casting practice, but that isn't what I want to know.

I want to know at what specific temperature CaSO4·2H2O calcines, that is, loses its bound water. Not recipes for heating molds with plaster(s).

And 140C to 180C mentioned above (if correct) is kind of a big range for a straightforward chemical reaction process, I think it's probably more specific.

Once I understand what the calcining temperature of plaster actually is, then I can move on to understanding other parts of the molding process, and truly WHY differing heat schedules are used by different people. But first things first. Lets get the basics down.

I do truly appreciate the responses, btw!


ps. I gotta say though -- the guy who did his master's degree thesis on investment casting -- that is one of the most complex, cost ineffective means of producing a rough aluminum casting with scabby finish I have ever seen!  I can straight sand cast a better looking part in a half hour with a simple charcoal furnace. Not trying to brag, here, just, that kind of complexity is absolutely unnecessary -- a 3D printer, 49% fused alumina content, 24 hours of fuel for kilning, ten risers and a tooled flask, sheesh!
« Last Edit: August 04, 2014, 07:38:54 AM by vtsteam »
I love it when a Plan B comes together!
Steve
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Offline awemawson

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Re: Plaster of Paris, Investment, etc.
« Reply #4 on: August 04, 2014, 07:27:58 AM »
Steve, I'm wondering if the range of values quoted is because in the real world the Plaster of Paris perhaps varies in composition, and the reaction is a bit more complex than the simple CaSO4 .2H2O -> CaSO4 + (free) 2H2O implies.

With a well regulated lab oven I suppose a series of experiments would fairly easily determine the value for a specific sample but it wouldn't necessarily be true for all sample of 'Plaster of Paris'

I used to set my kiln at 650 deg C, but what the temperature actually was, and whether it needed to be so high I have no idea !
Andrew Mawson
East Sussex

Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #5 on: August 04, 2014, 07:35:40 AM »
True Andrew, but I want to know what the basic chemical reaction temperature is, even if real world variations in plaster are a factor. I can extrapolate out for that, but I want a base understanding of the chemical process.
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #6 on: August 04, 2014, 07:47:27 AM »
Okay here's what I mean -- excuse the tech talk, but this is an example of how specific this kind of thing can be -- unfortunately it's not for plaster of Paris:

Quote
Calcination reactions usually take place at or above the thermal decomposition temperature (for decomposition and volatilization reactions) or the transition temperature (for phase transitions). This temperature is usually defined as the temperature at which the standard Gibbs free energy for a particular calcination reaction is equal to zero. For example, in limestone calcination, a decomposition process, the chemical reaction is

    CaCO3 → CaO + CO2(g)

The standard Gibbs free energy of reaction is approximated as ΔG°r = 177,100 − 158 T (J/mol).[3] The standard free energy of reaction is zero in this case when the temperature, T, is equal to 1121 K, or 848 °C.

Examples of chemical decomposition reactions common in calcination processes, and their respective thermal decomposition temperatures include:

    CaCO3 → CaO + CO2; 848 °C :
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #7 on: August 04, 2014, 07:55:29 AM »
Okay, so elsewhere in Wikipedia it says:

Quote
Gypsum plaster, or the plaster of Paris, is produced by heating gypsum to about 300 °F (150 °C):[2]

    CaSO4·2H2O + heat → CaSO4·0.5H2O + 1.5H2O (released as steam).

When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hours.[3] If plaster or gypsum is heated above 392°F (200°C), anhydrite is formed, which will also re-form as gypsum if mixed with water.[4]

A bit more approximate (about 300C) and the source cited is a dead link, so not entirely sure. And a new fact comes to light "anhydrite" which means no water? Not even the 0.5 people usually cite in the reaction. So there are two different calcining points? Unclear yet.
I love it when a Plan B comes together!
Steve
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Offline mattinker

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Re: Plaster of Paris, Investment, etc.
« Reply #8 on: August 04, 2014, 08:17:43 AM »
Okay, so elsewhere in Wikipedia it says:

Quote
Gypsum plaster, or the plaster of Paris, is produced by heating gypsum to about 300 °F (150 °C):[2]

    CaSO4·2H2O + heat → CaSO4·0.5H2O + 1.5H2O (released as steam).

When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hours.[3] If plaster or gypsum is heated above 392°F (200°C), anhydrite is formed, which will also re-form as gypsum if mixed with water.[4]

A bit more approximate (about 300C) and the source cited is a dead link, so not entirely sure. And a new fact comes to light "anhydrite" which means no water? Not even the 0.5 people usually cite in the reaction. So there are two different calcining points? Unclear yet.

I'm not sure I understand what you don't understand! I have found several sources (French plaster manufacturers) that say that the three out of four molecules of water are driven off at above 140 to 150°C, from what I can see, the Anhydrite CaSO4 is over 200°C The only thing I haven't found is what happens to plaster when heated over 200°C, does it too become Anhydrite?

I remember being told 'but I don't remember who by!) that making plaster was an irreversible process, I wonder whether the initial heat cycle prevents the loss of the remaining molecule of water.

Regards, Matthew

Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #9 on: August 04, 2014, 08:42:55 AM »
It is not a matter of "understanding" Matt.

Water boils between 200 and 250F, but even my 8 year old can get more specific than that.

And in fact, that specificity is important to an understanding of much in science and history that follows from it.
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Steve
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Offline awemawson

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Re: Plaster of Paris, Investment, etc.
« Reply #10 on: August 04, 2014, 09:07:39 AM »
I know I think where Steve is coming from - there are many woolly descriptions giving 'largely correct' facts, and he want's the actual facts in the sense of an accurate scientific approach that can later be built on.


BTW to play devils advocate I can boil water at FAR lower temperatures than you quote above (*)  :lol:


(* cf lousy cup of tea on the top of Everest)
Andrew Mawson
East Sussex

Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #11 on: August 04, 2014, 09:29:55 AM »
heh, Andrew, at STP and not yer muddy stuff!

I just realized I'm asking this question in the wrong place. I need to find a chemistry answers forum and just ask about the formula reaction. Back with that soon -- if I get an answer.... :coffee:
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Steve
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Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #12 on: August 04, 2014, 01:21:01 PM »
And here's the answer, attached.

And it shows the to changes and why there is a range, and where that range begins and ends and the proportions along the way.

And anhydrous definitely means no water.


I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline awemawson

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Re: Plaster of Paris, Investment, etc.
« Reply #13 on: August 04, 2014, 01:51:43 PM »
If only you'd asked - I'd have told you THAT ......   :lol:
Andrew Mawson
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Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #14 on: August 04, 2014, 09:50:08 PM »
So here's what we can say for sure:

Plaster of Paris (gypsum) when heated goes through 2 separate chemical reactions that release bound water. These are:

CaSO4 .2H2O --> CaSO4 .1/2 H2O
             and
CaSO4 .1/2 H2O + 1 1/2 H2O --> CaSO4 + H2O

The first begins as the gypsum is heated at 100C (212F) and continues until it reaches 150C (302F).

The second begins as the gypsum passes 150C, and ends when it reaches 185C (365F)

After reaching 185C it has been converted to anhydrite and all water has been released.


Now there are almost certainly other reasons for the claims of heating to 600F or 600C even, but they have nothing to do with the plaster investment conversion process.

What they probably have to do with is baking out the wax.

Which is the next topic I'm interested in.




I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline mattinker

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Re: Plaster of Paris, Investment, etc.
« Reply #15 on: August 04, 2014, 10:04:28 PM »
You are describing making plaster, the pre-heating of the mold is not about making plaster, but about preparing the mold for casting the metal. The reason that is given traditionally is that the three molecule of water need to be driven of right into the core of the investment. This is not powdered gypsum, but a mass of plaster and grog. It is altogether probable that a lower temperature can be used, but there is an enormous difference between 600°F and 600°C! There is possibly a question of thermal shock, fluidity also comes into play.

Regards, Matthew

Offline vtsteam

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Baking out the wax
« Reply #16 on: August 04, 2014, 10:40:10 PM »
Wax bake out:

Okay so we have another bunch of conflicting information scattered round the Internet about bake-out times. Some people bake their molds for a "few" hours, some for 24 hours, and some say 48 hours is essential.

Obviously the fuel and regulation needs for 48 hours while maintaining temperatures in the 600-650C range is considerable. So why the variation here?

How long does it take to get wax out of a plaster mold at over 1000F? Well, I imagine that if you subjected a candle to a 1000F atmosphere It wouldn't take but a few moments to go up in a flash. Something else must account for these times.

Maybe you need to increase temperatures of the mold slowly so it doesn't develop excessive stresses and crack, or explode from steam being boiled off in the plaster conversion process.

Well how slowly? I guess that depends on the mold thickness and composition. That warm up should finish around 185C (365F) when the plaster has gone anhydrous. There shouldn't be any steam after that. But maybe even anhydrite can develop stresses if heated too rapidly beyond this point. So maybe that accounts for some time.

Any residual wax in the mold will be molten at 185C though not necessarily burning. Depending on the wax, its autoignition point is about 225C-250C. So the remainder of the wax not poured off probably has absorbed to some extent into the mold, and hasn't been reduced to ash yet. Probably the less absorption the better. Which means the less wax residue after pouring out the pattern, the better.

Because if it absorbs it doesn't burn. No oxygen. Like a candle it needs to wick out of the plaster to reach oxygen. But when it does and burns, is that good for the plaster surface and detail.?I don't imagine it is.

So how to reduce the wax residue. Well, steam it out. And if that can happen, it will be happening at lower temps -- maybe even 100-150C during the plaster conversion process. If you could steam it all out, it wouldn't absorb, and burn later.

Now I have noticed in a few online comments that people recommend drying out a mold first before beginning the casting process. While others recommend the opposite -- keeping the plaster damp, or even soaking it.

Maybe the reason is that by doing that they can create enough steam to repel residual wax before it absorbs at a later stage in the curing process.
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #17 on: August 04, 2014, 10:45:53 PM »
You are describing making plaster, the pre-heating of the mold is not about making plaster, but about preparing the mold for casting the metal. The reason that is given traditionally is that the three molecule of water need to be driven of right into the core of the investment. This is not powdered gypsum, but a mass of plaster and grog. It is altogether probable that a lower temperature can be used, but there is an enormous difference between 600°F and 600°C! There is possibly a question of thermal shock, fluidity also comes into play.

Regards, Matthew

I am not describing making plaster. Yes I know there is a difference between 600F and 600C.  I did not say a lower temperature can be used to cast. I said that the chemical reaction completes at a lower temperature.
I love it when a Plan B comes together!
Steve
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Offline awemawson

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Re: Plaster of Paris, Investment, etc.
« Reply #18 on: August 05, 2014, 02:06:57 AM »
Steve
I always did three stages:

1/ steam bulk of wax with steam wallpaper stripper

2/ melt any solids left - kiln at about 200 deg C - wax drips onto baking tray and is removed from kiln

3/ burn residue - kiln ramps up to 600-650 deg C and held there for several hours dependant on bulk of mould.

I know that you want accurate parameters for the various stages but at a pragmatic level these three steps work
Andrew Mawson
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Offline Anzaniste

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Re: Plaster of Paris, Investment, etc.
« Reply #19 on: August 05, 2014, 02:48:37 AM »
Will a mould made from plaster of Paris be ok to use for casting a Babbitt metal component with a pouring temperature of 450C?
Scrooby, 1 mile south of Gods own County.

Offline RussellT

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Re: Plaster of Paris, Investment, etc.
« Reply #20 on: August 05, 2014, 05:49:45 AM »
Hi Steve

The reference you've found is interesting but an important aspect that will have a big influence on the converserion to anhydrous calcium sulphate is how fast the unbound water can be removed.

In the open pan trial it is all removed straight away - not surprising as they are using powdered material.  It will take much longer to remove the water vapour from the centre of a block of plaster - which will affect the reaction speed (and temperature) - as demonstrated by the pan with a tiny vent hole method.

Undoubtedly given long enough at 185 deg C all the water will be driven off, but it might be better to do it at a higher temperature as the increased vapour pressure should speed the process.

I am also wondering whether it is necessary to get a thick mould completely dry - if for example parts of a mold were a couple of inches thick then would getting the surface dry to a certain depth be OK.  I'm pretty sure it would if the mould is hot as there would be no chance of a steam explosion.

This might explain the variability in the casting procedures - it may be related to mould thicknesses.

Russell

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Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #21 on: August 05, 2014, 09:33:40 AM »
Andrew total agreement with you that an attachment to theoretical exactitude is foolish.  I'm just trying to understand what goes on behind the scenes for my own benefit. I don't like just following other people's recipes, I also want to know what's happening.

I find that if I understand that, I often can discover something new, or different, or when I make a change to a procedure it has a better chance of succeeding rather than being just a shot in the dark. That's the fun in it for me. Understanding what's happening beneath the surface.

I'm basically just thinking aloud but not coming to any practical conclusions here, at least so far. I haven't said what should be done in investment. I'm thinking about what has been done.

Anzaniste, re. babbitt I'm not the person to ask because I haven't done it, someone else who has should say. I have poured lead "ducks" (drafting spline weights) in a reusable plaster of Paris mold made without any heat curing or baking, but simply letting the mold dry for a week -- that was a loooong time ago. I'm not recommending no bake, just saying I did it at the time according to descriptions in a book. Lead has a lower pour temp.

And if people are also pouring aluminum in baked molds, and it has a higher pour temperature than babbitt, I would guess the answer is yes, it can be done.

RussellT total agreement with what you are saying about length of time to drive off water will be dependent on thickness. However We should be careful of whether we are describing the temperature something is subjected to vs the temperature it is at. The temperature vs free water graphs did not show time as a variable. vs thickness since that's dependent on so many different things.

However, we can say that water at 365F (185C) is steam at 300 PSI -- pretty obvious that any free water in the plaster in the size molds we are interested in here would migrate out nearly instantly, since plaster is porous. I don't imagine any free water remains at these temperatures. And they certainly don't drive water inward.

I am talking about plaster at that temperature, not surface temperature of a big block with a gradient inside at lower temperatures, or the temperature a kiln thermostat is set to.

So when people give procedures and temperatures, that's not what I'm talking about. I'm talking about plaster that has arrived at that temperature. And I'm not recommending any procedure at all here.

I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline RussellT

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Re: Plaster of Paris, Investment, etc.
« Reply #22 on: August 05, 2014, 10:13:17 AM »
I don't imagine any free water remains at these temperatures.

I think some will - as the plaster is heated steam will drive out the air - I don't see what will drive out the last bit of steam.  Given time diffusion will substitute air for steam - and to be pedantic it's still only at 15psi  - unless you can heat it fast enough to make the pores of the plaster into pressure vessels of their own.

I don't claim that the remaining water will have any significance at all - but I would be cautious about applying results from a powder to a solid plaster.

Russell
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Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #23 on: August 05, 2014, 11:43:33 AM »
Russell, sorry, I should have said 350 psi potential pressure.

If there is porous resistance, at 185C the pressure will increase internally until the vapor escapes, the greater the resistance the greater the internal pressure. If you say the steam escapes slowly, the resistance is correspondingly high then. Not 15 psi.

Here's the situation: either the plaster matrix is very porous and the pressure never gets to be much more than 15 PSI because the steam  all escapes. Or if not very porous, the internal pressure is well above 15 psi. The greater the resistance, the greater the pressure -- until it equalizes by complete release from the plaster.

In practical terms on the scale we're talking about, that shouldn't take long. Plaster is very permeable to steam, and even water, which is far less fluid. Cold water soaks readily into even air dried plaster -- in fact solid plaster of Paris is used to de-water clay in slip casting molds.

I don't think steam at 185C is going to reside long in an inch or two of plaster in a mold for a model engine part. This is what we're talking about. Not casting a life size horse and rider in bronze, right? Or at least that's what I'm interested in.

The speed at which steamcan be released is probably what determines how quickly the mold is heated initially, since building up pressure will cause it to break. That would be a big concern when the plaster still has unbound water at temps approaching and probably somewhat above 100C (since water can exist as a liquid under pressure above that point).

An interesting question (to me) would be seeing pressure curves as the plaster internal temp increases at various rates. In say 2 inches of plaster. And understanding at what internal pressure it breaks the solid.

Not that such information is available to look at, or even necessary to practical casting if you already have a process that works well for you. Just that I like to see stuff like that, and understand things.

It's just fun for me to think about how things work. It's my way of taking apart a clock, like when we were kids.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline RussellT

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Re: Plaster of Paris, Investment, etc.
« Reply #24 on: August 05, 2014, 12:46:22 PM »
Hi Steve

I understand completely what you're doing - and I enjoy reading about it.

I've always found that if you understand how and why stuff works then you have a much better chance of remembering it.

As a child did you ever try taking stuff to bits only to be working desparately to get it back together again before your parents came in.

Russell
Common sense is unfortunately not as common as its name suggests.

Offline awemawson

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Re: Plaster of Paris, Investment, etc.
« Reply #25 on: August 05, 2014, 01:32:14 PM »
Yes my late brother tried that approach with a frog, but it didn't work when he put it back together  :bugeye:
Andrew Mawson
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Offline mattinker

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Re: Plaster of Paris, Investment, etc.
« Reply #26 on: August 05, 2014, 02:30:47 PM »
Here's something to have under your hat for later, the microwave is a good way to accelerate drying of small pieces of plaster . I have not yet tried de-waxing using the microwave, it should be good as you can heat things up nice and slowly.

Regards, Matthew

Offline vtsteam

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Re: Plaster of Paris, Investment, etc.
« Reply #27 on: August 05, 2014, 02:44:02 PM »
That sounds good Matthew, and another interesting part of that is that it heats more evenly throughout. Rather than the heat having to penetrate through from the outside.

I remember from one of Steve Chastain's advanced casting books that plaster of Paris (is it okay if I just say poP from now on?) is used for pouring basin or sprue insulating liners in casting work. poP is I think the best insulator found for this.

So if it is such a good insulator, I imagine that would slow down regular convection heating even more, compared to microwave heating. That could account for a good chunk of the time required.

I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg