Author:  Created:  Rating: EJSAUNDERS Oct. 12, 2002, 07:10AM   11 Ratings     12345     Choosing Plasters If your a first time caster, you'll likely want to know what to buy to cast your blocks in.  As well as using the readily available Plaster of Paris, which can be found readily, available in hobby stores, DIY retailers or even your local supermarket (Wal-Mart US or Asda UK) there are other, specialist Gypsum products that have had their properties modified to be superior in quality for hobby casting. Many have additives which help maintain smooth-working qualities and upon drying, result in a hard, smooth working surface with reduced paint absorption and add a degree of chip-resistance without sacrificing any strength. A good article on plasters is available (plaster comparisons) which was submitted by Knightsfoil. It offers information on the standard range of plasters, commercial alternatives and industrial products. Sourcing some of the harder plasters may not be easy for some, so here's a few suggestions of places to source your plaster: * Your dentist * Dental Suppliers * Yellow Pages * Hobby Stores * Concrete/Tarmac Suppliers * Drywall Experts * Ceramics Centres * Builders Merchants * Home Depot (US) * B&Q/Homebase (UK) I am building up a list of plaster sources, which can be added to if you have any suppliers yourself, by clicking the Add You Link text in the top left corner.   US Gypsum US Gypsum (USG) manufactures a range of products that are specially designed for use in modelling. For superior chip-resistance and strength, USG produces a special plaster blend of polymer and synthetic fibres for producing solid, chip resistance and impact strength increased cast items. Their industrial product has more strength than USG White Art Plaster, USG No. 1 Casting Plaster or Hydrocal White Gypsum Cement. This product, Industrial Plaster PC, is recommended for casting whenever superior chip resistance is desired. These plasters are ideal for making items such as three-dimensional plaques, figurines, lamp bases and even simulated-stone furniture. The superior strength permits thinner castings weighing 25 to 35% less. White Hydrocal and Hydrocal B-11, by USG was suggested by Mick who found that the White Hydrocal was slightly stronger and had a quicker 'set-up' time, but was most impressed with the light weight and smoothness of the finished blocks. The only issue he found was that the B-11 product 'set-up' with a very flat and even center. The white Hydrocal tends to expand slightly with the addition of water and rises slightly in the center after scraping. Hydrocal White Gypsum Cement offers a gradual setting time and a long period of plasticity, so is recommended for both solid and hollow castings. It is especially designed for thin sections, which require high strength to minimize breakage during removal from an intricate latex mold. Sineater suggests their Drystone product that he assures is lighter than Diekeen and Excalibur and nearly as hard as either product. The product is slightly harder to mix and remains fairly hard and lumpy until thoroughly mixed, and then it will pour easily. The product dries fast and baking it in your oven produces a finished block with a similar strength to Excalibur. The only downside to the product is its inability to hold a very sharp, thin edge, so is more suited to those blocks without a sharp angle or edge. BMT Gander commented that it’s 9000 PSI air-dried, or 16000 PSI oven dried compressive strength was impressive, but is easily influenced by incorrect mixing. If mixed correctly, it should pose very few bubble problems, and has very little volumetric expansion. One of the hardest and strongest of all gypsum cements when mixed properly, is USG's Hydro-Stone. Although not suitable for carving, Hydro-Stone Gypsum Cement is recommended for producing high-quality novelty and statuary castings requiring extremely hard surfaces. Best results are obtained when this product is mechanically mixed. For those who want extreme durability, USG's solid casting, Tuf-Stone Gypsum Cement is a new, polymer modified, fibered material specifically manufactured for giftware applications. This product's unique formulation provides three times the impact resistance of standard plasters, greater chip resistance, and much higher resilience. BMT Gander suggests that USG's Hydrostone is much better for projects that are intended to be handled and is similar in most other qualities to Hydrocal. It is better at holding finer edges, but is also heavier by volume. He also suggests Ultracal 30, with a good compressive strength of around 12000 PSI, which makes a much stronger product than Hydrostone. Unfortunately, the product is slightly less able to hold fine edges, and is brittle in some cases. However, Gander suggests for static pieces, it's ideal, since it holds detail well and is tough with little volumetric expansion. For those who want DieStone with a little more strength, Gander suggests Resin Stone (RZ10, RZ15) that is a synthetic mixture of DieStone and resin fibres. However, since there is no catalyst, it's completely safe for use with RTV silicone molds and offers superior handling qualities and erosion resistance. Down sides of this is that it is difficult to get all the bubbles out, but not impossible for the experienced caster. Plaster of Paris Sineater compared Plaster of Paris to several other products, but he feels although its very brittle and weak, it remains a suitable plaster for beginners and casting thick blocks, such as floor tiles or regular blocks won't be a problem.  The other good things about it are that it is readily available and cheap to buy. Rusty agrees that the product is cheap and is a great starter plaster, which scrapes well and has a fair 2500 PSI compression strength when dry. He also suggests drying it in a dehydrator for 12 hours to ensure the product is dry throughout before use. Bad points of the product are that it is not the idea choice for castings with many undercuts or high detail. It is too thick to pour but mixes to a semi-smooth paste with low volumetric change. BMT Gander also agrees its a cheap, readily available product, that in his words "is very forgiving". Other properties of use in some projects are its easy to cut, carve or sand, but this also means that dropping a construction made in Plaster of Paris can easily break it. Dental Plaster Rusty considers the products very expensive, but they do scrape well (because it has a low volumetric change property), when mixed properly, which also makes a smooth, pourable paste. It has a high, 8000 PSI compression strength when fully dry, which means its ideal for detailed castings and comes in a myriad of colours. Sineater was very impressed with the Excalibur product which is the hobby version of dental plaster, but it is extremely costly and in very limited supply. BMT Gander comments that its high compressive strength of 18000 PSI, and the ease at which it mixes smoothly and pours well means its an excellent choice. It also holds detail to an unbelievably high degree, and has virtually no volumetric expansion. It can also be ordered in almost any colour imaginable, but comes in a standard four colours for hobby casters, available from his website. Die Keen BMT Gander is impressed with the compressive strength of 16000 PSI and the superior handling characteristics of Die Keen, which are very similar to Excalibur. Although the product is not officially available to the hobby industry, it can be sourced from dental labs if you have a nice local dentist, but it also costs (on average) half again as much to twice as much as its commercial alternative, Excalibur. Vatican Stone Cwaugh has used the product, and wasn't impressed because of the crystallisation of the product into a chunky plaster slurry. He comments it is very hard, even harder than Excalibur or other dental plasters, and has an interesting texture to it, which may be quartz embedded in it. Cwaugh also wondered if these sharp quartz pieces would damage the mold if in constant use. Lynchpt however liked the product, although comments that it is expensive, and other similar products may offer a more economical alternative. It was suggested though, that this premium price was reflected in the range of colours that the product can be bought in. It also has as said, a very interesting texture, which could save you time painting the structures. BMT Gander wondered if the product is modified Hydrocal, although this theory can't be proven because the manufacturer's are very closed-mouthed but a data sheet on Hydrocal's properties is available from the manufacturer's website. Durham's Sineater considers Durham's Rock Hard product as a good casting material, which he says is strong, but is very rough and porous, so absorbs paint more. It also takes a considerable length of time to set. Highland Piper also uses Durham’s Rock Hard and comments he has never had a problem with it being too rough and porous. He is also impressed that the blocks come out bubble free with a good strength to them, even though the product is considerably lighter than Plaster of Paris. Isa mixes a small amount of Durham's Water Putty into her plaster mix which makes the bricks stronger and also allows you to de-mold the blocks while they are still fairly soft. Isa suggests 1 & 1/2 cups of regular Plaster of Paris to 1/4 cup Durham's Water Putty. She warns that it may give you slightly grittier bricks and it sets up fast, so be ready to put your plaster paste straight into your molds or it may set in the mixing bowl. Using too much will also cause problems, most noticeably crumbly castings. Aside from the above differences, it does turn the bricks a slightly warm color so if you a darker block, add a tablespoon of hydraulic cement from your local DIY store or colour it using one of the methods below. Cements and Gypsum-based Hydraulic Cements Rusty reviewed Rockite patching cement (may either made by Walter Drake or Hartline), which according to the manufacturer "sets faster and stronger than concrete, holds better than cement". When properly mixed is very smooth, so can be poured like batter. Although Rusty comments it has a low volumetric change property the manufacturer admits it expands slightly when drying. Drying time is quoted as being "approximately within 15 minutes". Rusty considers the product expensive, and hard to scrape when it has been mixed thinly. He was impressed with the high 18000 PSI compression strength when fully dry, which makes it excellent for fine details. Rusty suggests the product is dry within 3 hours in a food dehydrator or low-temperature oven. He says the product is hard to find though had he has only been able to find it "at smaller hardware stores". Rusty has also tried three different brands of standard Portland cement. Two of them were quick setting but they still didn't set-up as quickly as regular plasters. And even when he tried to cut the cement with Plaster of Paris, although it set-up quicker, the results were not impressive. Some of the mixtures had coarse additives that made scraping a nightmare, and may damage molds if used for prolonged periods. Pour-Stone Pour-Stone, by Custom, is reported to be "Stronger than fully cured concrete", which sets in 20 minutes, and fully cures with an hour. Kittock also commented that the cured product is very strong and cannot be snapped readily with bare hands. Kittock, who reviewed the product, is unsure of the availability of the product outside of California (the manufacturer's website does suggest they have nationwide manufacturing facilities), so urges others to find similar products that contain the active ingredients of the mixture, which are cement, and gypsum. Knightsfoil suggests substituting the product for Ultracal 30, which is a blend of Gypsum, cement and silica, but Ultracal may be more expensive, since it is specifically produced for the caster, rather than Customs Pour-Stone, which is hydraulic cement. Accessories, Complimentary Products or Additives Several companies produce latex mortar mixer additives, which can be successfully added to any of the plasters above to strengthen the end resulting casts. The majority of these products are acrylic based, and a cheaper alternative may be created by adding Johnson & Johnson's Future Floor Wax (US, called Klear in the UK) or diluted PVA glue, which will accomplish the same thing. Both will increase water resistance and improve shock-absorption. Be careful when mixing in either product as the plaster will thicken and in some cases may set-up quicker in the pot. Future or Klear also needs to be stood for a minute or two once it has been whisked into the plaster due to its capacity to increase the amount of bubbles in the plaster mix. Mixing in PVA or acrylic binder will increase overall strength, as will adding sand, Kaolin or Fullers Earth (Clay/Cat litter), but a maximum of 20% additive to 80% Plaster of Paris mix is recommended. Other additives can be used to change the properties of any gypsum based plaster (all those above contain at least some gypsum), and I've found a handy PDF on gypsum additives, which is available on my site for reference. As the PDF suggests, adding a tiny amount of salt, to a solution of warm, not boiling water (57% max) will decrease the time the plaster takes to set-up, whereas adding borax to iced water will increase the setting time of the plaster. Polymer Additives Breaugh asked what plaster polymers were and wondered if they were likely to affect the mold like plastic casting resins do.  He also wondered if it would affect the final detail of the castings, how long it would take to cure and the overall strength of the finished cast. Jon Mines had a search online for information on polymer additives, and found some great links which are below: DuoMatrix-G (very informative link) www.smooth-on.com/dmtrxg.htm BMT Gander commented that iIf you choose to use duo-matrix, you should be careful with the finished product. The latex (essentially, just rubber) ties the structure together, but also inhibits drying within the structure itself and so the inner plaster and rubber will not dry out and a piece made of duo matrix which is broken some time later will emit a pretty serious stench. ZCrete - makes something called PolyBond www.zcrete.com/Default.htm A whole range of polymers, some of them look to be for gypsum related products. www.fitzchem.com/mfg_wacker.shtml USG products www.gypsumsolutions.com/a...0Additives After reading some of Jon's suggestions, I was intrigued about adding additives and being the sort of person who likes to test things out, I tried adding some bits and pieces: So, if we mixed a propionate amount of the following, we (in theory) would get a slurry, that when dry, would offer advanced weather resistance, be fairly water resistant, lightweight (more than regular Plaster of Paris) and very durable.  Plaster of Paris Cement Powder Fiberglass Filler Powder/Strands/Flaked Sheet Latex Milk/Acrylic Gel/Paint Crystalline Silica (Powdered Sand/Silica Desiccant Salt water to reduce set-up time (since adding additives may mean the set-up time will be longer) My original results seemed to indicate that the blocks would require a considerably longer drying time (even with the salt additive).  Within 8 hours, the pieces were still very wet and could be easily crushed between finger and thumb or scratched with a fingernail. However, within 24+ hours, the smae piece couldn't be scored with a fingernail and whacking it with a small jewelers hammer or large pliers didn't seem to dent or flake it at all. Some of the other castings in the same group were  not so fortunate though. I constructed several pieces in various ratios of additive to plaster, and the following occured:: More PVA (my acrylic polymer) in the mix - the block remains very wet after +-24 hours of drying and did not dry at all.  The conclusion I came to was that excessive use of additives and not enough water and plaster means the chemical reaction that hardens the castings could not happen.  Similarly less plaster in the mix had the same effect.  Again, the conclusion is that adding more plaster would hopefully remedy this. So I deduced that if you add anything you will need at least over >75% of the mix to be plaster and water.  I would recommend that the best ratio is 90% plaster and water and <25% additive.  This mixture produces a hard, smooth, and finely detailed block. In the end I used the following additives: Acrylic Polymer - PVA Glue Fiberglass Filler - err, I used sawdust Silica - Silica desiccant Cement Additional - Aluminium powder (Alum BP) Artists Graphite powder Fiberglass filler can be found in car repair or automotive stores, DIY or home improvement type stores and even in some good hobby stores.  Silica desiccant is basically powdered sand, and can be found in good electronic engineer's supply shops or anywhere that sells electrical equipment that may be affected by moisture (such as a camera retailer).  If you can't find any I achieved similar results with plain sand, although the overall 'roughness' of the piece did increase.  Cement is available at all home improvement (DIY) stores and local builders merchants.  Aluminium (Alum BP) can be found in good chemists and graphite powder can be found in good art shops. You can also search Google for chemical suppliers, who should be able to sort you out with powdered silica, aluminium and graphite powders. Colouring Plaster There are numerous ways to colour your creations before painting, that is, to colour the original casts so they don't have to be painted perhaps as much as a standard 'white' plaster brick would or won't look quite so bad if you miss patches or where a piece has broken or chipped off. Many of the users of Hirst Arts molds suggest: * Food Colouring * Latex House Paint * Acrylic Model Paint * Cartridge Pen Ink * Windsor and Newton Acrylic Ink Note: When mixing the colour, add the dye to the water and not to the mixture. Also remember that if you add more liquid you will dilute the plaster so substitute colour for water. The more colour the darker the blocks will be (don't forget most plasters are pale and will require a fair amount of colour to be dark when dry BUT will be slightly darker when completely dry). Shadetree had success with adding Apple Barrel's craft paint to his plaster - available in Wal-Mart in the US, or similar quality paint available in the UK is Inscribe. Shade also commented that the bricks seem no weaker than standard blocks, and have a less grainy and smoother feel that takes paint well. Adding acrylic paint to your plaster mix, is much like adding the Future or Klear or PVA, it mixes well with the gypsum and creates a stronger, more flexible casting (you can't bend it but it is less prone to flaking or breakage). Prakseda has had similar success with adding the tinting colours range from Valspar, which can be found in most local hardware stores in the US. Nyteseer has also had success in using powered dyes for plaster and says that it doesn't take a great deal to get a good tint on the plaster, and come in a variety of colours. Although Nyteseer suggested his powder dyes affected the setting times of the plaster, I have successfully used children's powder poster paint in my mixes, which have had no detrimental affect on setting times.  Public Comments Please log in to post a comment    Created  Comment  Jun. 30, 2004, 05:57PM  By: HAWKWIND I have been using Durham's Water putty exclusivly for about six months. I love the way it can be additionally textured, is easily sanded and carved. Modifying bricks to suit need is very easy, but the matrial is still very strong. The color is nice as well. small pieces can simply be sealed and left a nice golden wheat color, like a light sandstone. A touch of ink and they look fabulous. Its also very easy to get, keeps well, and is inexpensive. I'd like to try dental plaster at some point, but I'm pleased as punch with what I'm using for now.

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