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Casting Problems - Alginate, Plaster & Painting

Most casts will have little imperfections in them, but they really are not that noticeable once painted and it will still be a most beautiful keepsake. 

The information covered in this guide is organised in three sections:

1. MAKING THE MOULD
2. WORKING WITH PLASTER
3. PAINTING & FINISHING THE CASTS

1. MAKING THE ALGINATE MOULD

Misshapen Moulds

This are two reasons why this can happen:

  • The subject moved too much during the moulding process. If the subject is proving wriggly whilst trying to create a mould, wait until they are still or fast asleep. 
  • You have used too much water or water that is too cold. BabyRice Skin Safe Alginate should be mixed using equal volumes of alginate and 30ᐤC water. Excess water or cooler will retard the setting process and can ruin the mould.

The alginate took a really long time to set

There are several reasons why this can happen:

  • The water is too cold. BabyRice Skin Safe Alginate is formulated to start setting at around 120 seconds when mixed with water 30ᐤC water. Cooler water will result in a slower setting time. 
  • Too much water has been used. BabyRice Alginate should be mixed using equal volumes of alginate and water. Excess water will retard the setting process.
  • Poorly mixed alginate will have an uneven consistency which will set unevenly.

The alginate sets too quickly

There are several reasons why this can happen:

  • The water is too warm. BabyRice Skin Safe Alginate is formulated to start setting at around 120 seconds when mixed with 30ᐤC water. Warmer water will result in a faster setting time. 
  • BabyRice Alginate should be mixed using equal volumes of water and alginate. Using less water will speed up the setting process, and can even cause the material to begin the setting process before you have finished mixing it.
  • Always sprinkle the alginate powder into the water for mixing (not pour the water into the alginate) - this is especially important for when mixing a large amount of alginate with water, like for adult or family hand casting.
  • Poorly mixed alginate will have an uneven consistency which will set unevenly.
  • Taking too long to mix will make it seem as though the alginate is setting quickly.
  • Do ensure the thermometer you're using is accurate for measuring water temperature

The alginate didn't set

There are a couple of reasons why this can happen:

  • The water is too cold. BabyRice Skin Safe Alginate is formulated to start setting at around 120 seconds when mixed with 30ᐤC water.  Very cold water will not make the alginate set.
  • BabyRice Alginate should be mixed using equal volumes of water and alginate. Using excess water will make the alginate/water mix too sloppy and it will not set.

Bubbles in the mould

Bubbles in the mould result in small, protruding bubbles on the surface of the cast as the plaster fills the space inside the bubble.

While it’s almost impossible to prevent air entering your alginate mix, there are several reasons why you might find excess bubbles in the mould:

  • The alginate mix is too thick. BabyRice Alginate should be mixed with an equal volume of water. Using less water creates a thicker mix, which is likely to trap air. 
  • Poor mixing technique. BabyRice alginate should be mixed gently with a spoon or spatula, using a firm circular motion. It should never be whisked as this introduces more air into the mix. 
  • To mix a large quantity of alginate (approx 1kg -1.5kg) an electric drill with a paint mixer attachment can be used. 
  • Protruding bubbles can be easily picked off the cast while the plaster is still damp.

The alginate is lumpy

When fully mixed with water the moulding material should have the consistency of a thick porridge. If the alginate/water mix is lumpy this will affect the quality of your final cast. There are several reasons that can cause your alginate turn lumpy:

  • Poor mixing technique - i.e. mixing too slowly or adding water to the alginate instead of adding alginate to the water. 
  • The mix is too thin - i.e. too much water.
  • The mix is too thick - i.e. not enough water.
  • The water is too hot - i.e. warmer than 30ᐤC - causing the some of the mix to start setting.

Extending the life of the mould

Fill the mould with your casting plaster as soon as possible and always within 15-20 minutes. If you cannot cast your mould immediately, you can extend its life by carefully wrapping the mould in a damp cloth, placing it inside a sealed container and storing it in the fridge for up to an hour (max) before pouring in the plaster.


2. WORKING WITH CASTING PLASTER

Excess plaster in crevices of the cast

This is caused by defects in the mould. To prevent this:

  • The subject must keep still during the mould making process. 
  • Ensure the alginate is thoroughly mixed. Poorly mixed alginate will have an uneven consistency which will set unevenly.
  • To remove excessive plaster from the surface of your cast:  Gently 'pick' off excessive plaster while the cast is still damp, using something like a cocktail stick.
  • Once the plaster is dry, use a card-style nail file, trimmed to size, and sand away any plaster you do not want.

Air holes on the surface of the plaster

This is down to either a poorly mixed plaster or poor technique for filling the mould. 

To minimise air bubbles/holes appearing on the surface of your cast, try the following:

  • Ensure your mix isn’t too thick.
  • When mixing plaster and water, stir it slowly. You are not agitating it, as if whisking eggs. Stir firmly, as if stirring paint that had not been used for several weeks. 
  • Fill the mould slowly and gradually, tipping and rotating the mould in several directions as you go. 
  • Once the mould is full of plaster, tap the sides of the container.

Missing parts of fingers or toes

This is caused by poor technique - when you filled the mould with plaster, an air pocket was created at the fingers, instead of being filled in with the plaster. To prevent this you must:

  • Spoon in a very small amount of plaster gently rotate the mould. Repeat this process one spoon at a time until the mould is full. 
  • When casting feet, tilt the casting container while the plaster sets. 
  • It's possible to build back up these areas with a fresh plaster. Of course, you will not be able to replicate details such as fingerprints.

Finger or toe snapped off while demoulding

There are two reasons why this can happen:

  • Poor demoulding technique - refer to accompanying instructions for more information on how to demould your cast. 
  • Using a poor quality or soft plaster - always use a strong plaster, such as Herculite II, which is included in all BabyRice casting kits.
  • How to fix a broken finger or toe: Wait until the cast is thoroughly dry and use superglue to reattach the affected digit. If you have a little plaster left over, you could apply a small amount to seal any visible crackline. Once painted no-one will notice the crack!

Lines on the surface of the plaster cast

These lines are caused by:
  • too much water in the plaster/water mix;
  • poor mixing technique;
  • or working with the plaster too slowly.
The correct water to plaster ratio is 1:2. The plaster will start to settle to the bottom of the water quite quickly, so it's important to work qickly. If the plaster starts to settle before you have finished pouring the plaster into the mould, you will end up filling the mould with watery mix. This is what creates the appearance of 'watery' rivulets on the surface of the cast.

You can rescue those cute casts by gently brushing over the surface with a thin layer of wet plaster, making sure to wet the surface of the cast so that the new plaster can bond with the old plaster. Once the fresh plaster is dry you can gently sand any rough edges. Once painted you really won't see it!

Parts of cast lacking definition or detail

There are two reasons for this, one is movement when the alginate was setting around the hand/foot OR, the plaster has not fully filled in these parts of the mould. To prevent either of these:

  • Make the mould when your baby is fast asleep to avoid any movement in the alginate mould
  • Spoon in a very small amount of plaster and gently rotate the mould. Repeat this process one spoon at a time until the mould is full. 
  • When casting feet, tilt the casting container before the plaster set an keep the container tilted.
  • To repair, once the cast has fully dried lightly sand the toe nails. Once the cast has been painted it will look fabulous!

3. PAINTING & FINISHING THE CAST

Paint is thin and watery

Stir or shake the paint thoroughly before use. Please ensure the lid is firmly closed prior to  shaking. 


Metallic paint disappears / is the wrong colour / is glittery / is matt

The reason that the paint hasn't taken to the cast is due to the cast still being wet/damp inside. Casts should be allowed to thoroughly dry out for at least 5-7 days before painting and ever a longer wait time is needed for larger casts or if left in a damp or humid environment.

The paint won't adhere to the cast when it's still damp, so the paint appears to disappear into the plaster. When using metallic paints, this can leave you with a cast that appears silver – this is because the metallic particles are left on the surface while the paint pigment disappears, literally being sucked into the plaster.

How do I remove the paint from my cast?

You cannot remove the paint from your cast. Acetone will remove some of the paint layer, but it will dampen the plaster, which will also remove the detail in the plaster cast and ruin it. 

The metallic wax has dried out

Over time, the metallic wax can become solid as it’s exposed to the air. To soften, add a few drops of water and reseal the pot. Leave overnight or until the wax has absorbed the water. Repeat as required until the wax has the required consistency.