I wasn't going to make this a monthly column -- others are more pressing -- but after spending a couple of hours on it I figured it counts.  But I'm not counting it for the monthly schedule.  ;)  It's just a bonus.  Once I get a little more time in with my numerous brushes, that column(s?) will appear soon.

I recently received an email inquiring about what plaster I would recommend for casting.  I've used several over the years and I thought I had some records of their various properties.   Seems I can't find anything in the 'ol computer but I did find that I have had this question asked of me many times over the years.  :)  So I decided to go out and collect the info as a resource for everyone.  

I won't go into the technical composition of plasters such as 'what is a 'gypsum' plaster or how to mix or use plasters.  But I will include some helpful links below.  If you would like me to expand on this column to cover some of that information, just let me know.  I'm happy to add it.

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There are bascially two main considerations for choosing a plaster: hardness and set up time.  Hardness doesn't exactly equate to 'chipping resistance' but it's seemed like a good proxy for me in the past.  

Hardness is rated as 'compressive strength' in psi.   Setting time can vary a fair bit depending on ratio of water added, temperature of water, volumn of plaster being casted, etc.  So consider these times as a general guide only.  

Setting time does not equal 'fully cured' NOR 'working time' so consider that.  The full hardness is not reached until it is fully cured which can take a day or more depending on the volume of the cast and the ambient temperature and humidity.  I found my cast blocks took a few days to fully dry -- piled in a box -- and you will know when it is cured by the sound when you drop a piece.  It has a ping-y sound that is quite different from the duller sound it has when not fully cured.

These numbers below are derived from technical sheets where possible, Bruce Hirsts page on plasters, and a quote from a Woodland Scenics product rep. for Super Strength Plaster.  (I trust Bruce's numbers for the most part.)

So, without further ado here is a list of (almost) all the plasters I know of, along with their hardness and setting time. Question marks indicated I don't really know; it's a guestimate.  List is in increasing hardness:

Plaster of Paris ------ ~2,500 : 5-10 min
Woodland Scenics  Mold-A-Scene ----- 1,500? : 10min?
Woodland Scenics Carving Plaster ----- 2,500? - 10 min?
Woodland Scenics Lightweight Hydrocal ----- 4-5000? : 10 min?
HydroCal  -----  5,000 : 25-35 min
UltraCal 30  -----   6,000 : 25-35 min
Flowstone  ------  9,000 : 25 min?
Woodland Scenics Super Strength Plaster  ------   ~10,000 : 20 min?
Tufstone  ------   10,000 : 25 - 35 min
Hydro-Stone ------ 10,000 : 17-20 min
Drystone  ------   10,500 : 25 min?
Merlin's Magic  ------   14,000 : ~15 min
Diestone  ------   14,000 : 20 min?
Excaliber/Die Clean  ------   18,000 : 20 min?

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I've used UltraCal 30, Tufstone, Merlin's Magic, Hydro-Stone and only dabbled with WS Super Strength.  I find any of these more than suitable for casting Hirst Arts blocks.  I wouldn't go below the 5,000 psi mark for anything that will be handled during use.

When buying plaster, shipping can be a problem.  Big bags are heavy...  Some only come in larger amounts like 50 lbs and that is more plaster than most people will need for their lifetime.  Over all my casting history, I think I went through 3, maybe 4 50lb bags - and I cast A LOT of Hirst Arts blocks over the years.  :)  Check to see if distributors are nearby for a local pickup.  

Note that Tufstone is fiber reinforced.  When it does crack, those fibers to help hold it together and it resists chipping as a result.  But they also stick out a bit from the casts and while very tiny, you can see them.  They aren't a real problem when painted and only show on the scraped/poured surface.

Also note that WS Carving Plaster is designed to be soft enough to be carved for rock textures or similar -- I suppose brickwork might be an option???.

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Resources:

Woodland Scenics Plasters
Lists plasters and tools with chart to 'guide' selection.

Hirst Arts Casting Page
Describes tips for casting some plasters and links to suppliers

Hirst Arts Casting Page - Users  
Has testimonials for several plasters including ones I didn't list above.

Plaster Master
Crazy looking site but has some really interesting information and tips for casting with plasters.  Includes links to suppliers.

--- See attached for some USG Technial Data on their plasters, and WS guide to their plasters.