Plaster casting à la Linka

[spikey]

Can anybody please tell me what type of plaster I need for casting in Linka-type moulds?  I have need of rather a lot of retaining wall, and casting it seems to be the most cost-effective solution - but not if I have to buy expensive plaster!

[Mark Saunders]

Last time I cast any I was just using Plaster of Paris and that worked fine.

 

Mark Saunders

[Andy Hayter]

Plaster of Paris will certainly do it.  If you can get hold of dental plaster (which I think has a few bits added to Plaster of Paris) this will give a slightly more durable casting in my experience.  This is important for exhibition layouts where jarring during moving and transportation can lead to cracking.

[Damo666]

This may sound silly, or obvious (but it wasn't to me at the time).

 

Make sure you use the correct mix of water to Plaster of Paris and leave to set at a cool room temperature. I ended up with brittle castings (too much water which weakened the mix) and banana shaped casts (dried out too quickly as I was impatient and tried to dry them near a radiator). Also had bubbles in some mixes but I cannot remember the reason why.

[runs as required]

I have had to make fairly large (3ft x 4ft 6in) castings for murals etc. which I have cast either face down against a mould or textured surface, or face up which I work on with scoring tools when the mix is on the point of going off.

 

Plaster of Paris or dental plaster would be too expensive for these quantities. So I use general purpose builders' plaster and depending on the amount needed, I mix with PVA or lime to give a degree of elasticity. (like classic lime mortars for stonemasonry need to be elastic).  For large areas I don't want to crack, I embed fibres into the piece (e.g. bits of sacking). At other times weight gets critical so it is good to put styrene in to save weight by creating voids.

 

As others have said, it is best to let such large pieces cure slowly overnight.

dh

 

Edit

One thing not mentioned in the above posts is about mixing:

First of all always use a clean plastic washing bowl or similar for mixing - and always add the plaster to clean water you have placed in the bowl beforehand.

This is often hard to judge, a tendency is to overdo the water and end up mixing a uselessly runny 'slurry'.

Edited October 2, 2017 by runs as required

[spikey]

Thank you, gentlemen.  That's me sorted.

[Theakerr]

Don't know what happened to my post yesterday but I will try to repeat from memory.  Basically the best stuff I have found is Dental Plaster.  Comes in big bags but you can often scrounge a bit from your local dentist.  It is strong and hard enough to be filed/sanded and the particulate is very small so the detail is good.  I have had some success with "casting powder" available from a good craft store but it is soft and not as strong as dental plaster.  I have not been at all happy with Plaster of Paris.  It is soft, breaks easily and has a relatively course particle size.

[PatB]

I make plaster moulds as part of my living. I use what is sold here as "superfine" plaster which I buy in 25 kg bags from a ceiling specialist. There are a couple of brands available here, one of which is a bit crap and the other is very good. A bag costs ~AU$25, so it's not the end of the world if it doesn't all get used before it goes crunchy. I don't know if it's as fine or as strong as dental plaster, but it's fine enough to pick up brush marks in the release agent I use between the wooden masters and the newly poured plaster. Strengthwise, it's hard to tell, because the work I do isn't really directly  comparable to model building construction, but it seems quite durable, with an individual mould providing ~50 porcelain slip-castings before it starts to show major deterioration.

 

Getting air bubbles out is a combination of giving the mixing bucket a good jiggle before pouring (bubbles will congregate in the middle of the surface and can be skimmed off with an old spoon for particularly critical applications), getting a hand into the poured plaster and running fingers around all the areas where bubbles might stick while it's still liquid, and finally thumping the underside of my moulding bench with a rubber mallet for a minute or two to settle thing down. One day I'll buy/build a vibrating table to do the same job.

 

Plaster is stiff enough to remove the masters (for me, not necessarily for what you're doing) after about half an hour, depending on ambient temperature and humidity. However, full cure takes much longer (days) but, in my case, the dryness of the mould is more critical. By the time it's dry enough to use it will be fully cured. Again, cure time may be more important for Linka type castings.

 

I have noticed that plaster I've had lying around for a while tends to go off more quickly than fresh. I've just been using a bag I've had sitting around for 3 months, not having done much moulding recently, and it's been kicking off while I've still had my arm in the mixing bucket, which is a bit disconcerting. Makes it rather exciting as to whether I'm going to get it poured and debubbled in time .

[Macbear]

I make plaster moulds as part of my living. I use what is sold here as "superfine" plaster which I buy in 25 kg bags from a ceiling specialist. There are a couple of brands available here, one of which is a bit crap and the other is very good. A bag costs ~AU$25, so it's not the end of the world if it doesn't all get used before it goes crunchy. I don't know if it's as fine or as strong as dental plaster, but it's fine enough to pick up brush marks in the release agent I use between the wooden masters and the newly poured plaster. Strengthwise, it's hard to tell, because the work I do isn't really directly  comparable to model building construction, but it seems quite durable, with an individual mould providing ~50 porcelain slip-castings before it starts to show major deterioration.

 

Getting air bubbles out is a combination of giving the mixing bucket a good jiggle before pouring (bubbles will congregate in the middle of the surface and can be skimmed off with an old spoon for particularly critical applications), getting a hand into the poured plaster and running fingers around all the areas where bubbles might stick while it's still liquid, and finally thumping the underside of my moulding bench with a rubber mallet for a minute or two to settle thing down. One day I'll buy/build a vibrating table to do the same job.

 

Plaster is stiff enough to remove the masters (for me, not necessarily for what you're doing) after about half an hour, depending on ambient temperature and humidity. However, full cure takes much longer (days) but, in my case, the dryness of the mould is more critical. By the time it's dry enough to use it will be fully cured. Again, cure time may be more important for Linka type castings.

 

I have noticed that plaster I've had lying around for a while tends to go off more quickly than fresh. I've just been using a bag I've had sitting around for 3 months, not having done much moulding recently, and it's been kicking off while I've still had my arm in the mixing bucket, which is a bit disconcerting. Makes it rather exciting as to whether I'm going to get it poured and debubbled in time .

 

I love using Linka and plaster. It can be frustrating if  you try to get the castings out of the moulds too quickly or slowly after pouring, and on no account try to speed things up by putting them on a radiator... I bought mixtures from Linkaonline see http://www.linkaonline.co.uk/store/c1/Featured_Products.html and there is a decent beginners' guide to plaster: http://www.linkaonline.co.uk/all-about-plaster.html  Curiously, the main man seems to get a tad fed up with mixing up his own recipe dry plaster mix, but he certainly has expertise!

Sorry not to have found the query earlier and I hope this may still be of use.

 

My old stock of plaster has been good for many years. Yes, I've started on new stuff as I've recently come back into making my own layout after a good 15 years - it's now for me not the children. A good airtight seal to the storage bin and only opening in a room with low humidity is possibly important - old snap on buckets from DIY projects with sealed plastic bags inside etc seem to do the job.

 

Glue - good old white PVA is fine, but for rapid bonding bits I like UHU. The casts must be properly dry and with large areas a backing such as a cut out from an old cereal box applied with PVA is strong.

[MikeTrice]

I was told many years ago by a Linka demo person to use Artex.

[Martyn T]

I was told many years ago by a Linka demo person to use Artex.

Artex used to contain asbestos.

[Nearholmer]

For a previous H0 layout, I needed about six feet of wharfside, about 3” high, and made a master for a short section, and a latex mould. The plaster came from an art and craft shop (a proper one, not a ‘warehouse’) and was very good, fine and very hard. It took detail very well, including grain on the wooden things (rubbing strokes?) that are part of wharves.

[Dave 46]

Artex used to contain asbestos.

 

But hasn't for decades.  I recently sampled some Artex ceilings from the 1960's - no asbestos detected.

Modern Artex is no problem.

 

Dave Smit

 

[polybear]

The world is full of asbestos.  Whilst not applicable now, think of all those worn out brake linings and the dust they produced in the process.

[Mark Saunders]

But hasn't for decades.  I recently sampled some Artex ceilings from the 1960's - no asbestos detected.

Modern Artex is no problem.

 

Dave Smit

 

Modern is a bad term as my house was built in 1976 and has trace levels of white in it!

 

After the mid 1980's this was banned but obviously stocks remained till used!

[Dave 46]

Modern is a bad term as my house was built in 1976 and has trace levels of white in it!

 

After the mid 1980's this was banned but obviously stocks remained till used!

 

Sorry for the late reply, I've been without a computer for a few days.

 

By "modern" I meant current.  I assumed that what was being talked about was using new Artex bought from a builders merchant now.

 

Dave