6.4. Plaster of Paris
The word “plaster” means a substance which, like clay, can be moulded to almost any shape when soft, but either dries or cures to a hard material. Clay and cow dung mixtures used for surfacing walls and floors are called “clay plaster”, and this material simply dries (with some shrinkage) to a hard material. However, it can be made plastic again by adding water. Lime plaster and gypsum plaster have the special quality of “curing” after they are mixed, which means they undergo a chemical reaction that makes them hard, so that even if placed in water, they will not become soft again. On heating to 120ºC gypsum releases about 20 % of its weight as water. This chemical reaction can be written thus:
2(CaSO4.2H2O) - 2CaSO4.H2O + 3H2O
In words: gypsum + heat = plaster (and water which goes into the air).
When the plaster is mixed with water the process is reversed and the plaster returns to its original gypsum state and becomes hard.
“Plaster of Paris” is the type of plaster used for making moulds for ceramics. It was developed in France around 1770, which gives it its name. With the use of plaster of parts, the ceramics industry was revolutionized, because it made possible mass production by slip casting, which was previously unknown. Plaster of parts has the special property of easily absorbing large quantities of water, and there is no other material that can be substituted for slip casting.
The raw material for making plaster of parts is gypsum, which is hydrous calcium sulfate (CA2SO4.2H2O). Gypsum deposits are found in most countries. Very simply, making plaster is done by grinding the gypsum, and then heating it until chemically-bonded water evaporates at 110ºC. This is similar to the process of making lime, which is known in most countries. It can be done on a very simple level, but industrially is done on a large scale and controlled in a sophisticated way.
6.4.2. PLASTER OF PARIS PRODUCTION
It is normally not worthwhile to produce your own plaster if ready-made plaster is available. In several countries this is not the case and the potter may have to make his own plaster from gypsum. Often he will be able to sell his plaster in the local market to other potters or to manufacturers of writing chalk.
Gypsum is a soft rock that may occur as transparent or slightly white to grey crystals, and sometimes as fibrous granules. Several geological varieties exist: selenite is a clear crystal; satin spar is white and fibrous; alabaster is massive and normally white; and gypsite is a mixture of gypsum and white sand. The crystal form can be scratched with a finger nail, which produces a white line. Gypsum is also used by the cement industry, and in some places farmers use it as a source of calcium for the soil. Deposits of gypsum are rarely pure and the gypsum content may be only 65 % of the bulk material, the rest being impurities of lime, clay or sand. The purer the raw gypsum the better the finished plaster product will be. If possible, check the deposit of gypsum before taking delivery and explain to the supplier that you only want the clear crystals.
Old plaster moulds can be ground and recalcined, since already-set plaster is gypsum (chemically speaking). This should only be tried if raw gypsum is not easily available or very costly, since the quality will not be as good as with freshly-calcined gypsum.
Production moulds get contaminated with deflocculants and clay. First the moulds are cleaned of clay, dust and deflocculant crystals deposited on the mould surface. The moulds are then crushed and the fine lumps are washed in water, which will remove the soluble deflocculants. After drying, the material is processed as described below. The quality of recycled plaster is not as good as plaster made from raw gypsum and the extra cost of cleaning the old plaster should be compared with the cost of fresh raw gypsum.
First, wash away sand, clay and other impurities from the raw gypsum. The gypsum is then pulverized in a hammer mill. Smaller quantities are simply heated in a pan and when the gypsum starts to boil it is stirred gently until all of it has been boiled. It is difficult to judge when the gypsum has been heated enough - it is better to overheat slightly.
first & second set
Gypsum starts boiling around 120ºC s. Original S. 112 and more water is released.
Plaster that has been through this “second set” is less plastic than “first set” plasteron the other hand it is stronger. With simple equipment like a pan it is not possible to control when the “first set” is over, because some of the plaster in the bottom of the pan starts boiling again before all gypsum has been through the “first set”. Generally, it is better to have some “second set” plaster, rather than risking uncalcined gypsum, which will weaken the plaster moulds and shorten the time it takes for the plaster to harden after mixing with water.
For making larger quantities, a kettle for calcining can be made locally. A cylindrical pan made of brickwork or sheet metal is set inside an oven heated from below. Mechanical agitators stir the gypsum to ensure even heating. A thermometer inserted in the gypsum enables the operator to know how the calcination is progressing. At around 120ºC boiling starts and the temperature in the upper layers will drop to around 100ºC until the first set is finished. Then the temperature will rise again to 170ºC when the second boiling starts. If heating is too fast, it may be difficult to recognize that the boiling of the “first set” has stopped before the second starts. In Burma we went through the “first set”, and after seeing the temperature rising again we heated until the gypsum started to boil. We then let this second boiling go on for about 15 minutes (600 kg batch) before ending the heating. The agitators were left to stir for another 20 minutes to allow all moisture to evaporate.
screening & storing
After cooling, the plaster is screened through 60-100 mesh and packed in bags, which are stored in a dry place. If the plaster becomes moist it will set and be spoiled in the bag.
Freshly-boiled plaster requires more water to become fluid, resulting in moulds that are porous and weak. After some weeks ageing the plaster will absorb a little moisture from the air and this will enable a plaster mixture to be produced with less water. For this reason it is better to store the freshly calcined plaster for 1-4 months before using it. In industrial production, a small amount of dissolved deliquescent salt is added to the plaster during calcination and this has the same effect.
Plaster is widely used in industrialized countries. Large-scale plants either use big kettles holding up to 20 tonnes or rotary kilns. In principle the process is the same as the one taking place in a simple pan, only much better controlled and yielding a plaster of consistent quality.
Plaster is often available with different setting times. For example, U.S. suppliers can provide “20-minute casting plaster” or “30 minute casting plaster”, and offer more than 30 different types of plaster and gypsum cement. For specialized model making, these products are often useful.
Setting time can be retarded and speeded up by addition of chemicals. Retardants are sodium carbonate, vinegar, dextrin. Accelerators are warm water and salt. For specialized model making, this is often useful, but normally it is better to use the plaster without any addition since this reduces its strength. Small amounts of uncalcined gypsum will speed up the setting time. Shorter setting time is a sign that your plaster is not stored properly and has become too old.
A special plaster is produced by calcining the gypsum under pressure in an autoclave to 120ºC. The calcining thus takes place under humid conditions and this produces plaster with a special crystal structure (alpha plaster) that makes the finished set plaster much stronger, as more plaster can be added to the water. This type of plaster is, of course, more expensive, but the extra cost is justified when used for model making and for block and case mould production. It is essential for moulds used in the ram press.
A) The raw gypsum is cleaned.
C) Gypsum is changed to plaster in a calcining kettle.
E) Screened plaster is packed in bags.
6.4.3. SHIPPING AND STORAGE
Because plaster of parts is easily damaged by water, care should be taken with shipping and storage.
If you must order plaster of parts in some quantity, it is better not to order more than 6 months’ supply at a time. Request the supplier to provide it in polythene-lined bags, or in sealed drums. Plaster should be stored above the floor: bags may be placed on top of boards, to allow air circulation below and prevent any water from getting into the bags. It should not touch the wall. It also should be stored so that the oldest plaster is used up first.
6.4.4. MIXING PLASTER OF PARIS
When mixing plaster, it is very important to always use the same amounts of water and plaster powder. If more water is used, the strength will be less and the water absorption greater. Less water will make a thicker mixture, which will be stronger and which will have less water absorption. This will cause problems in the slip casting section, with moulds casting at different speeds.
plaster to water ratio
The strongest mixture of standard quality pottery plaster is 70 parts (by weight) water to 100 parts plaster. This is used for case moulds. Working moulds used for jiggering and casting are mixed with 75 - 80 parts water to 100 parts plaster. Where plaster is expensive it is better to use a high proportion of plaster, because the working moulds will last longer.
Plaster used for bats or dewatering trays are mix with 90 parts water to 100 parts plaster.
Gypsum cement (alpha plaster) can be mixed with only 40 parts water to 100 parts plaster. This produces a very hard plaster with high expansion and low absorption, ideal for master and case moulds.
The right ratio of water to plaster cannot be fixed. It will vary from one source to the next and from batch to batch. So every new batch should be tested and compared with the last batch.
rules for plaster work
RULE 1: ALWAYS WEIGH THE PLASTER AND THE WATER !
Most mould makers get lazy after a time, and start to just estimate the amounts. Sooner or later, they will make mistakes, and the casting section will have problems. PROBLEMS COST MONEY!
RULE 2: NEVER PUT WET HANDS OR TOOLS IN THE PLASTER BAG !
Always use a dry bowl or spoon when taking plaster from the bag. Any moisture will cause the plaster to set in the bag.
RULE 3: AFTER STARTING TO MIX, NEVER ADD MORE WATER !
After you start to mix plaster, the chemical reaction begins immediately. Sometimes, if the plaster starts to get thick too soon, the mould maker will try to add water to make it thinner. This doesn’t work - the mould will be soft and may never set.
RULE 4: DO NOT PUT AIR BUBBLES IN THE PLASTER !
Air bubbles are Enemy Number One of the mould maker. If they are on the surface of the mould, the quality will be very bad. Follow the instructions below for mixing plaster very carefully. Air bubbles do not come from the plaster - they come from the hands of the mould maker.
Here is the detailed work process for mixing plaster of paris:
For good quality moulds, it is important to have clean tools. Always use a clean container to mix the plaster. Plastic buckets are best, because they are easy to clean. Dirty tools and mixing containers will result in poor quality moulds. Tools should be washed immediately after use.
Always weigh the water first, and then add the correct amount of plaster by weight. Experienced mould makers think that they can estimate amounts correctly. This is not true: usually mistakes will be made, and the quality of moulds will be uneven. Temperature of water is also important. Cool water means slow setting; hot water means fast setting.
Plaster should be sifted into the water through a coarse sieve to remove big particles.
4 mixing time
The time for mixing and pouring should always be the same. After plaster is added to water, it should be left without stirring for 2 minutes. This allows the plaster particles to absorb water. At the same time, air bubbles rise to the surface.
5 by hand or machine
Hand-mixing is satisfactory for amounts under 3 kg. Larger amounts should be powermixed. The mixer should force plaster from the bottom tip-not suck air into the mixture.
In hand-mixing plaster, it is important to stir the plaster quickly, but not to mix air into the plaster. First put your hand slowly into the plaster. All mixing should be done from the bottom with the fingers. The hand should not come above the surface of the plaster.
6 pouring plaster
Pouring the plaster into the mould should be smooth and steady, so that air cannot enter. It is best to place a small board in the mould first, and to pour the plaster over it - this spreads out the stream of plaster without introducing air bubbles. Experienced mould makers hit the table with their body while pouring plaster - this helps to release bubbles. The best way to remove bubbles is by using a brush, which is gently moved about the surface of the model immediately after pouring in the plaster.
7 setting of plaster
Plaster goes through several stages of becoming hard:
- liquid: As the chemical reaction starts to occur, the liquid will become gradually stiffer.
Remember that it is easiest to clean up plaster before it gets hard. Discard any excess plaster in a suitable container or old plaster bag. Buckets and tools should be washed immediately in a bucket of water always kept ready for that purpose. NEVER put plaster in a sink or drain, as this will block it.
Old, hardened plaster that falls from tools or buckets into fresh plaster will speed up the setting time, and cause bad quality moulds.
A) Measured amount of water is filled in the mixing bucket.
D) The mixture is stirred for 2 minutes. Stir with upward movements that release air bubbles.
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