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Oct. 12, 2002, 07:11AM
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Casting
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Pphalen, who had recently started casting asked
how experienced caster knew they had a good or bad casting since he didn't
have any significantly noticeable problems, such as trapped bubbles.
The only noticeable problem was the bottom of his blocks were concave due
to problems scraping, and wondered if perhaps using a larger scraper would
solve the problem of the scraper catching in the depressions of the
mold. He also wondered whether it was better to pull with the
scraper, or push and whether it made a difference at all.
As Shadetree responded, usually if a casting looks good, it is good, but
scraping can certainly alter the 'fit' (in regards to other blocks in a
structure) and the overall average height of blocks.
He says he had the same problems with scraping when he first started out,
and found that if you hold the scraper at a more acute angle (like
25° instead of 45°) and gently scrape (as opposed to really pull with
the scraper) you find that the blocks will turn out smooth, evenly level
and almost perfectly flat on the bottom. Shade suggests that the
only way to become perfect with the process is to practice and any pieces
that you bungle too much can always be used as ruins later on.
I posted that the three main problems are that the blocks can often be a
bit dusty/crumbly, so check if they are hard or dusty (which can affect
their ability to stick together). To remedy, use more powder and follow
the directions as best you can on the package, since not all plasters are
mixed the same, some require almost exact water to powder ratios.
The other common problem is not enough powder or over-scraping, both of
which can lead to blocks with a dip in the center of the bottom (i.e. are
concave), so you get fine, sharp and brittle edges, but no middle in the
bottom of the block. To remedy, as Shade says, don't push so hard if its a
scraping problem, and if its a lack of plaster, just add some more.
Lastly, if you seem to have very watery blocks, this is again caused by a
lack of plaster powder, adding more, until you get a creamy, pancake-batter'ish'
fluid will reduce the water on the top (or bottom when done) of the
blocks.
In terms of the push-pull problem, I always tend to pull towards me,
from the one end to the other, then do it backwards (don't push, reverse
the stroke so that your more pulling with your hand bent). Then I tend to
go from side to side, trying to ensure that I don't make a mark in any of
the blocks with the point of the blade of my scraper.
Sineater adds that if your using Plaster of Paris, the water to plaster
ratio is pretty critical, and practice is the only way to perfect
this. If you have too little plaster to water, your blocks will be
very brittle and easily damaged.
Fellyo gave good advice that you shouldn't be discouraged if your
casting isn't perfect at first, or if pieces don't fit together perfectly,
you will learn with practice and patience.
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The 'Sports Bottle' Method
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Ellender suggest using a water sports bottle to pour the plaster paste into the molds. He has successfully cast all of his molds using this method several times without having any bubbles.
The sports bottles he is referring too are the type that have the little pop-up spigot on top, allowing you to drink without spilling, which are common in both the US and UK, and a general brand available in both countries is
Naya. You could use almost anything similar, such as ketchup or
sauce bottles available in good supermarkets or even a cake decorating
piping bag.
In his own words, here is the method he uses: "I mix the plaster in a bowl and then when I have the right
consistency I transfer it to the bottle, shake the bottle to fully mix it and then fill my molds. It has certainly reduced the time I take to cast my
blocks".
Knotweaver who also uses the sports bottle idea, has added markings on the side of the bottle for set quantities of plaster and mixes the plaster merely by shaking the
bottle which may assist in removing some of the problems associated with
getting bubbles in your castings. The only point to note is that may
need to add a metal nut or something to shake all of the plaster,
which will get stuck in any nooks or crannys at the bottom of the bottle.
He has found that occasionally, he gets a few lumps and air bubbles, which are usually negligible
and cmbining this with the glass "no scrape" method (see below), he finds that he gets very consistent blocks with few air bubbles.
He even achieves great results from the wooden shingle mold! |
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Reducing Bubbles
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When Martin casts, he tends to use his kitchen counter which he gives a quick spray and then sprays his molds. He then shakes off the excess "wet water". After filling his molds, for which he uses a regular spoon, he taps the sides of the molds with the spoon or a scraper. Since they are sitting on the damp surface, they move a bit and he says this allows trapped air to escape much easier.
I tend to use a large (rubbery plastic) sculptors mixing bowl and a 20ml syringe (no needle) and fill each brick up until its level at the top and wait 5 minutes for the pieces to be hard enough to de-mold.
This reduces the possibility of bubbles, is easier than using a spoon to
get the paste into the mold and is quick.
As a side note, you will usually only get water on
the top (the bottom of the brick when de-molded) of most plasters if you've not added enough plaster to the mix. Ensure you can mix the plaster right by
filling the block up to the top without overfilling or needing to scrape
and seeing how much water collects on the top. Keep practicing this
until you don't get any water on the top (you will need to try adding more
or less plaster until you've got it perfected).
Archangaeli also uses the syringe method proposed by Trollstomper, and has found that his casting process has really come together. Using a custom-made vibrating table,
Windex for the 'wet water' tension reducer (see below) and a syringe has almost removed the whole problem of trapped air
bubbles for him.
He found a good pharmacy that sells syringes of every size and he finds that using a 60ml size syringe means he can 'pour' an entire mold at once. He has found that using the syringe means the plaster flows down the length of the mold spaces better than when he spooned the plaster in. This he says, seems to help avoid bubble formation.
He also noticed that he seems to be using significantly less plaster overall with the syringe since he has better control over how much he adds to each mold and he can get the bricks level, meaning less wasted plaster.
An indirect benefit, he also notes that he has been able to measure more precisely how much plaster fills each mold, and with the syringe, that volume has been much more consistent from one casting to the next. The result is
an exact knowledge of how much plaster to make, so less waste again.
The downside to using a syringe is that the syringe must be rinsed between each casting and scrupulously cleaned at the end of each session to keep plaster from hardening inside the syringe and causing problems. |
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The 'Wet Water' Method
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Gamer suggests using the 'wet water' method of casting, which is putting some dishwashing
detergent or soap into a bucket of water, which is then applied to the mold
by dunking it into the bucket or by paintbrush or spray bottle before pouring in the plaster.
Some people use the brush, others dip the mold in or even leave their molds in the solution until they need them. This gets rid of air bubbles and makes the difficult pieces much easier to cast by reducing water tension.
As
a side note to new casters, the spray bottle method may be the source of
additional bubbles and so should not be used for casting blocks that may
be affected by this, such as the wooden shingle mold, the wooden plank
mold, or any blocks that are intricate or feature deep undercuts that may
trap bubbles.
BMT Gander, a great advocate of this method related this story "I got in
a hurry last night and relearned how important the wet water thing is. I poured
six molds without dipping them. When I started to thump I noticed far fewer
bubbles coming up. That's when I realized my mistake. Virtually every block has
some sort of imperfection. The single biggest piece of advice I can give is to
always dip the molds in soapy water before you pour. It's probably the most
important factor behind the plaster mix itself to making good blocks". I
personally find that by mixing the plaster correctly (if its got directions,
obey them!), I don't have to thump the table, mop up the water from the top of
the mold or get any bubbles out (I do use a syringe to put the plaster in the
mold though). |
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Scraping
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I use a 5" decorators wall paper scraper, but many fellow Hirst Arts followers, have used a variety of objects, some of the more successful are plastic rulers, any sort of paint scraper, or almost anything that is firm with a straight edge.
Shadetree suggest being relaxed and reducing pressure on the blade - just drag it over, rather than pull, which stops the scraper entering the block depressions and removing too much plaster. He also suggests that holding the scraper at a more acute angle (25° instead of 45°) improves the scraping of the mold.
I always pull the scraper towards myself, from the one end to the other, then do it backwards (don't push, but reverse the stroke so that you pulling the scraper with your hand bent towards your wrist). Then I tend to go from side to side, trying to ensure that I don't make a mark in any of the bricks with the point of the blade or remove any more paste than necessary.
NR Hussar Goat suggests a few helpful hints that he uses in the scraping and molding process are to:
* Weigh out ingredients.
* Make sure my work surface is level
* When lifting off the paper towel make sure you don't have plaster paste stuck too it
* tap the mold to allow the excess water to rise.
He also suggest that if your an inexperienced caster, you do a couple of trial passes with out removing any material so you can get the action and angle correct before doing it for real.
Gamer, amongst a lot of others tends to use the glass plate method: This is basically using a piece of glass, plexiglass or even a sheet of heavy plastic that is placed flat on top of the mold and then weighted to
expel excess plaster. This replaces scraping and makes for more consistent blocks. Takes practice to get all the air bubbles out though and to ensure the blocks are straight.
As Che LLe found out, occasionally adding your tissue paper to soak up any excess water from the top of your mold will result in a fair amount of water AND plaster being sucked up onto the tissue.
Wig was also having this problem when he first started, and found the best solution is to fill three or four molds at a time, making sure to overfill each cavity. He then waits about 6 minutes after filling the last mold, then gently puts a paper towel over the first mold, and lets it settle until all the paper in contact with the mold is wet. He then slowly removes the towel, starting with the edge closest to his and finds most if not all of the plaster stays on the mold.
If you continually have problems with very watery bottoms on your bricks (the top whilst casting) then it may be your not adding enough plaster, or too much water to your mix. If your plaster has directions, try to follow them, or if it hasn't a good rule of thumb is three
spoonfuls of plaster to one water and work out the best ratio of plaster to water for your plaster from that ratio.
BMT Gander replied that several molds have a tendency to catch the
scraper blade and so you should ideally try doing the initial scrape more vertical
than 45, and then (after wiping the scraping blade) run back at more like
a 20 degree angle.
BMT suggests the convex bottoms come from scraping at too severe an
angle. You're pushing the plaster down into the mold, and it's coming back
up when you're done. The more perpendicular angle (with very light
pressure) will push the plaster along, but not in. If you back
scrape almost laying the scraper down, it'll run some plaster back into
the slight dips you made with any catches of the balde and even the whole
thing out. So, the trick with scraping (although I'm by no means an
expert) is to scrape both ways. The initial scrape will generally give you
the disruption, but if you lightly run back in the other direction with
the scraper a lot closer to parallel than perpendicular, it'll generally
level things back out.
BMT Gander also comments that if you mix the plaster thick enough you won't have any trouble with water rising after you've placed the glass in the 'glass method'. He recommends that you try to achieve a 'thin pudding' look to your plaster
Kaag finds that using the 'glass method' means his blocks all turn out a little tool tall, and when creating structures with both stacked and on-end bricks, the stack always seems to turn out higher than the pieces on end.
Psaitta comments that he only uses the glass method and has found that the average thickness of the blocks are 6 thousands of an inch bigger, and bases his results on comparing his 8" round tower to the one Bruce has constructed.
Paul also commented that the true benefit of the glass method is speed, since you do not have to wait to scrape and most of the blocks will be the same level, so you can go on casting the next mold or mixing more plaster. Combining this method with 'wet water' really gets production up, especially if you have several molds and lots of room to spread them out in.
I personally have tried all the methods available for casting and can safely say that if you ensure you mix the plaster just right there is no need to scrape or use glass.
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The 'No-Scrap' Method
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| Matthew Freyman found that he needed to apply a
fair amount of effort and time to get the correct result when scraping and
if the plaster wasn't the right consistency, he found that when you scrape
water will continue to rise and form the concave appearance that alters
the size of your blocks and makes getting even walls a challenge.
Matthew decided he would prefer to try the 'no-scrape' method, which
seemed an easier way of getting perfectly flat blocks.
Paul (psaitta) was the original creator of the idea and here's an
introductory text on the method:
You will need a flat piece of Plexiglass (clear acrylic sheet) or thick
strong glass. As Paul says, be careful with glass because it will
cut you!
Fill the mold as usual and tap the side to get out bubbles. Then over fill
the mold a little and then slide the glass down over the top of the mold,
from one side to another, which should push out some plaster. Make
sure its completely covered with plaster residue and this should ensure
you will get flat, bubble-free blocks. When the glass is fully in
place, slide the glass around a little and it will get out most of the
excess plaster. Don't press too hard or you'll shorten the blocks or
deform them. You only want to get out the extra plaster out, not
cause any suction that will pull in air bubbles.
Let you blocks harden for about fifteen minutes, then you should be able
to pop them out scrape off all the excess plaster and start again.
Doing this method you get a perfectly flat bottom over all pieces.
Scooter, who also uses Paul's method finds that he gets problems only
if he messes with it after he's laid the glass down. His best casts,
are when he simply lays the glass down and slides it around slightly and
forgets about it. He tends to add a small weight onto the top to
avoid any possibly vacuum. He says that the only time he gets
miscasts are when he's messed around trying to remove trapped
bubbles. Things like sliding the glass around too much, lifting and
reseating it or pressing too hard cause more problems than just placing it
down and leaving it. He says if you find yourself doing that, give
up on it and either pour in some more plaster and replace the glass, or
if it's not too late, even it out with conventional scraping and try
the glass again the next time around.
Further information and a basic 'how-to' can be found inside Paul's
website: http://www.miniaturesetc.com |
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| Public Comments |
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I have found a way!
1: pour the plaster
2: take saran wrap (cling wrap) and lay over the mold
3: hold one end of the saran wrap down
4: Scrap the mold as you did when you first got the mold and used plaster of Paris, by using the saran wrap it levels out the top for a smooth mold and the fibers do not drag.
5: place a piece of glass/plastic on top of the saran wrap, fold the edges over and “slide” the glass (with the saran) off the mold, (instead of just lifting)
6: place the mold where you want it to dry
7: slightly tap the mold to remove any surface bubbles.
Ok, the reason you have to use glass/plastic is if you just slide the saran it will cause surface wrinkles. The reason I don’t just use the glass, is that I found it leaves a lot of surface air bubbles. Now, for the crazy part.. I got the idea from my cooking experience, when you make pudding and not want it to have a slime top you place saran on top of it to keep air from getting to it.
Robert
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I am trying TUFSTONE, but when I scrap it the fibers pull more plaster out of the molds and make them shallow, I am going to try the glass approach, but any ideas would be helpful
Robert n Texas
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