Matt's Replica Police Box Lamp

[matt sanders]

Hi folks,

Back in the summer, I started building a replica of the last surviving Metropolitan Police Box, which now resides at Crich Tramway Village - see my build diary in this section. 

As it is difficult to continue building the large components in the middle of the winter, I am currently concentrating on the small details, and first up is the Lamp.

As this is rather convoluted, and may be of interest to a wider audience than my main build, I'll start this special thread for the Lamp Replica.

Firstly here are some photos of what we are aiming for.....





So - let the fun begin!

[matt sanders]

Episode 1:   The Seeds of Dome!


Back in the summer I went to Crich a couple of times with Mark Johnson, and took some measurements of the Lamp.  Once back home, I carefully drew it up at full size.  This allowed me to make some mould-boxes out of foam-board, which I took to Crich in November, along with a bucket of silicone...

Here's the mould-box for the Dome, wedged into place with the vertical pieces of foam-board, and then with silicone poured over the dome...


When I was sure the silicone was covering the whole dome, I added this foam-board "lid", sealing the edges with tape, and filled it up with more silicone...


Once the silicone was set, I removed the foam-board, so was left with the silicone mould.

[matt sanders]

Back Home:

So now I've got a silicone mould of the Dome, but I can't cast anything useful from it, because it is soft and floppy.  So I need a rigid box to keep it in the correct shape.  The original mould-box was destroyed when exposing the silicone on site, so here's a new (matching) box, to support the mould.


When on site, I could not mould the underside of the Dome, because the Lens is in the way, so here's the next best thing:

1:  Cut a ring of foam-board, with holes in the correct places to receive the metal rods.  Cover the surface with Jesmonite resin (an acrylic liquid mixed with plaster-like powder), imitating the texture of the Dome's other surfaces.


2:  From my diagrams, I've worked out how thick the metal must be in the Dome, so using the cardboard jig shown here, I roll out plasticine to the correct thickness, cut it into hexagons, and lay it into the mould.


3:  The plasticine is smoothed together, to make the inner surface of the Dome.  It's not perfect, but it will hardly be seen on the finished Lamp.  Plus, I'll have a chance to sand it smoother, on the final casting...

[matt sanders]

Inner Mould

There's no picture of this, but to make the mould for the Dome's inner surface, I paint a couple of thin coats of silicone over the plasticine & Jesmonite.  (First painting vaseline on the original silicone mould, to stop the new silicone from sticking.)

This new silicone mould will be even more floppy, so to support it, I make a rigid jacket of plaster, reinforced with woven glass-fibre cloth:


The Plaster has set, so is lifted off, and shown here on the left.  Note how thin the silicone is.


The plasticine & foam-board are easily lifted out (shown on the right), leaving the two halves of the mould on the left.  Now it's ready for casting...

[matt sanders]

Casting the Dome:

First paint on polyester gelcoat, died with black pigment...


Reinforce with glass-fibre matting, with polyester resin...


When that has set fairly firm, blob plenty more gelcoat around the edges that will be joined together...


Close the two halves of the mould together, and leave them pressed together by something heavy, until the gelcoat has set...


Open up the mould, to reveal the casting...


Voila!


So - that was the easy part.  What's next...?

[domvar]

Once you have done with that mould how would you feel about pulling a couple of concrete ones from it ?

[matt sanders]

Well, I could do that, but the real dome is made of cast iron... ;-)

[matt sanders]

Episode 2:    Base under Siege

Now, obviously it would be quite tricky to make a mould of the Lamp's square Base, because it has four metal Rods and a Lens fixed into it.  But luckily, living inside the Crich box is a spare Lamp Base, that was thrown in by Hendon police training centre, when they donated the box in 1982.  It has lain in the dust for 32 years, but now it's ready for its close-ups!

[matt sanders]

Moulding at Crich:

First, I block in the big round hole, with foam-board and gaffer tape, and stick double-sided tape around the edge...  (And yes, this the desk inside the police box.)


As with the Dome, I've made a foam-board box at home, so now I stick the Base down inside it...


I mix up the silicone, and pour it over the top, to fill up the box.


A few hours later, the silicone is set, so I pull it off the Base, and take it home.  And that's where the trouble starts...

[matt sanders]

Back Home:

Here's the silicone mould - oops, it pulled off some of the flaking paint(!)


I clean up the mould, and keep the paint flakes safe, as they give handy colour references, not just for the dark Royal Blue, but also the paler blue that was previously painted on this base.  (The dark blue on this Lamp Base is probably our best reference for the colour of a 1960s police box, as, unlike the Crich Box itself, it has never been repainted.)

However, you'll notice that the sides of the rubber mould have a tendency to bow inwards, so I need to make a new box to hold them steady...


The box is folded around the silicone, and drawing pins hold the silicone sides in place.  Here, I'm painting in the black gelcoat...


Then resin & glass matting...


And now it's set!


I open up the mould, and find that the casting is slightly distorted, with its side walls still bowed in, despite my attempts to restrain them.  Looking back, I realise it was a mistake to make the original mould box from foam-board, because the hydraulic pressure of the silicone that I poured at Crich caused the walls of the original mould box to bow outwards.  So, when I try to enclose the mould in a new (unbowed) box, it distorts the shape of the mould within.  

So - back to the drawing board...

[domvar]

Well, I could do that, but the real dome is made of cast iron... ;-)

Ah interesting I had never got close enough to tell in which case how about plaster so a cast iron one could be made up later?

[matt sanders]

Well, if the aim is to make it in iron, then perhaps what is needed first is a casting of the dome in wax, which could then be used in a lost-wax casting procedure...?  I don't know much about that process, but this mould could certainly be used in some way for that...

[matt sanders]

Take 2:

This time, I make a new mould-box from rigid melamine-faced chipboard, and make a temporary square cardboard structure inside the mould, to ensure it will give an accurate square Base.  I wedge the silicone against this cardboard former, and pour resin around the outside, to create a perfect fit for the silicone mould.  (All this hassle could have been avoided, if I'd made a rigid box in the first place, before going to Crich.)


Cardboard ring and drawing-pins restrain the silicone in position, ready for casting...


As my full police box won't be ready for some months, I want to use this Lamp as a table-lamp, and perhaps make copies for friends.  So, to give a nice smooth surface on the inside, I stand it on each edge, and pour in more resin, allowing it to set before turning to the next edge.


Then I lay it flat, and pour in more resin, to give a flat interior surface.  (It's a cold winter night, so I use extra catalyst, to speed up the setting of these various stages...)


To be on the safe side, I leave it in the mould overnight, before de-moulding.


Yay - it worked!  This time the sides are straight!

[matt sanders]

Moving On...

So, I clean up all the edges on the Base, and drill holes for the metal Rods.  (Note that I've sanded the Dome's interior, to eliminate imperfections from the plasticine...)


I paint on grey primer...


But wait a minute - the Base's side walls don't look as straight as they did when it came out of the mould...  The next day they had warped even more - grrr!  I think it's because I used extra catalyst to combat the cold weather, but anyway, this Base is now also useless!

Back to the drawing board again...

[matt sanders]

Take 3:

So - fibreglass can't be trusted in this cold weather, so what am I to do?

Luckily, I also have Jesmonite resin, which is an acrylic liquid that gets mixed with powder, and can be died with black pigment.  The big pluses are that it produces no fumes so can be used indoors, it is not sensitive to cold conditions, and it produces very stable castings that simply do not shrink or warp.

Let's give it a go!


First, a layer of black resin is allowed to harden in the mould.  Note that this time I'm using woven glass cloth, which is especially made for this resin, and is much nicer to use than that spiky scratchy fibreglass stuff...


Again, I pour in more resin, turning the mould, to give a flat surface on each interior face, finally ending up with a nice neat result.


So, I take the finished casting out of the mould, and it's perfect - nice and rigid, and no distortion.  I carefully sand and finish all the edges.  The only disappointment is that there are a few air bubbles on the inner surfaces. 

Not to worry - I have some Fastcast - a two-part polyurethane resin.  I wrap gaffer tape around the edges of the casting, and pour some Fastcast on each interior surface.  But the air trapped in the Jesmonite's surface bubbles rises through the Fastcast, leaving bubbles in that surface too.  Oh well, it was worth a try - I'll just have to use some filler on the little holes.

But then when I remove the gaffer tape, I realise that Fastcast has also leaked around to the exterior surfaces of the casting, and ruined the moulded texture in numerous places.

You'll just have to imagine how this looked, as I was so pissed off that I slung the thing out in the garden, without taking any pictures.

Back to the drawing board again...