diff --git a/api/schemas.yaml b/api/schemas.yaml
index 7141cbb..3be9b11 100644
--- a/api/schemas.yaml
+++ b/api/schemas.yaml
@@ -111,7 +111,7 @@ Material:
           - $ref: 'api.yaml#/components/schemas/Color'
         properties:
           number:
-            type: number
+            type: string
             example: 5514263423
 
 Condition:
diff --git a/src/models/material.ts b/src/models/material.ts
index a5378e0..71d6b34 100644
--- a/src/models/material.ts
+++ b/src/models/material.ts
@@ -9,9 +9,9 @@ const MaterialSchema = new mongoose.Schema({
   carbon_fiber: String,
   numbers: [{
     color: String,
-    number: Number
+    number: String
   }],
   status: Number
-});
+}, {minimize: false});
 
 export default mongoose.model('material', MaterialSchema);
\ No newline at end of file
diff --git a/src/routes/condition.spec.ts b/src/routes/condition.spec.ts
index 5881e4b..8b4a73c 100644
--- a/src/routes/condition.spec.ts
+++ b/src/routes/condition.spec.ts
@@ -333,7 +333,7 @@ describe('/condition', () => {
     });
   });
 
-  describe('POST /condition/new', () => {  // TODO: sample number generation
+  describe('POST /condition/new', () => {
     it('returns the right condition', done => {
       TestHelper.request(server, done, {
         method: 'post',
diff --git a/src/routes/material.spec.ts b/src/routes/material.spec.ts
index 0faf04e..746a620 100644
--- a/src/routes/material.spec.ts
+++ b/src/routes/material.spec.ts
@@ -1,8 +1,8 @@
 import should from 'should/as-function';
+import _ from 'lodash';
 import MaterialModel from '../models/material';
 import TestHelper from "../test/helper";
 
-// TODO: numbers with color only (no number)
 // TODO: deal with numbers with leading zeros
 
 describe('/material', () => {
@@ -21,7 +21,6 @@ describe('/material', () => {
       }).end((err, res) => {
         if (err) return done(err);
         const json = require('../test/db.json');
-        console.log(res.body);
         should(res.body).have.lengthOf(json.collections.materials.filter(e => e.status === 10).length);
         should(res.body).matchEach(material => {
           should(material).have.only.keys('_id', 'name', 'supplier', 'group', 'mineral', 'glass_fiber', 'carbon_fiber', 'numbers');
@@ -35,7 +34,7 @@ describe('/material', () => {
           should(material.numbers).matchEach(number => {
             should(number).have.only.keys('color', 'number');
             should(number).have.property('color').be.type('string');
-            should(number).have.property('number').be.type('number');
+            should(number).have.property('number').be.type('string');
           });
         });
         done();
@@ -63,7 +62,7 @@ describe('/material', () => {
           should(material.numbers).matchEach(number => {
             should(number).have.only.keys('color', 'number');
             should(number).have.property('color').be.type('string');
-            should(number).have.property('number').be.type('number');
+            should(number).have.property('number').be.type('string');
           });
         });
         done();
@@ -85,7 +84,7 @@ describe('/material', () => {
         url: '/material/100000000000000000000001',
         auth: {basic: 'janedoe'},
         httpStatus: 200,
-        res: {_id: '100000000000000000000001', name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: 5514263423}, {color: 'natural', number: 5514263422}]}
+        res: {_id: '100000000000000000000001', name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: '5514263423'}, {color: 'natural', number: '5514263422'}]}
       });
     });
     it('returns the right material for an API key', done => {
@@ -97,6 +96,15 @@ describe('/material', () => {
         res: {_id: '100000000000000000000003', name: 'PA GF 50 black (2706)', supplier: 'Akro-Plastic', group: 'PA66+PA6I/6T', mineral: 0, glass_fiber: 0, carbon_fiber: 0, numbers: []}
       });
     });
+    it('returns a material with a color without number', done => {
+      TestHelper.request(server, done, {
+        method: 'get',
+        url: '/material/100000000000000000000007',
+        auth: {basic: 'janedoe'},
+        httpStatus: 200,
+        res: {_id: '100000000000000000000007', name: 'Ultramid A4H', supplier: 'BASF', group: 'PA66', mineral: 0, glass_fiber: 0, carbon_fiber: 0, numbers: [{color: 'black', number: ''}]}
+      });
+    });
     it('rejects an invalid id', done => {
       TestHelper.request(server, done, {
         method: 'get',
@@ -130,7 +138,7 @@ describe('/material', () => {
         auth: {basic: 'janedoe'},
         httpStatus: 200,
         req: {},
-        res: {_id: '100000000000000000000001', name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: 5514263423}, {color: 'natural', number: 5514263422}]}
+        res: {_id: '100000000000000000000001', name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: '5514263423'}, {color: 'natural', number: '5514263422'}]}
       });
     });
     it('keeps unchanged properties', done => {
@@ -139,10 +147,10 @@ describe('/material', () => {
         url: '/material/100000000000000000000001',
         auth: {basic: 'janedoe'},
         httpStatus: 200,
-        req: {name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: 5514263423}, {color: 'natural', number: 5514263422}]}
+        req: {name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: '5514263423'}, {color: 'natural', number: '5514263422'}]}
       }).end((err, res) => {
         if (err) return done(err);
-        should(res.body).be.eql({_id: '100000000000000000000001', name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: 5514263423}, {color: 'natural', number: 5514263422}]});
+        should(res.body).be.eql({_id: '100000000000000000000001', name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: '5514263423'}, {color: 'natural', number: '5514263422'}]});
         MaterialModel.findById('100000000000000000000001').lean().exec((err, data) => {
           if (err) return done(err);
           should(data).have.property('status', 10);
@@ -159,7 +167,7 @@ describe('/material', () => {
         req: {name: 'Stanyl TW 200 F8'}
       }).end((err, res) => {
         if (err) return done(err);
-        should(res.body).be.eql({_id: '100000000000000000000001', name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: 5514263423}, {color: 'natural', number: 5514263422}]});
+        should(res.body).be.eql({_id: '100000000000000000000001', name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: '5514263423'}, {color: 'natural', number: '5514263422'}]});
         MaterialModel.findById('100000000000000000000001').lean().exec((err, data) => {
           if (err) return done(err);
           should(data).have.property('status', 10);
@@ -173,20 +181,30 @@ describe('/material', () => {
         url: '/material/100000000000000000000001',
         auth: {basic: 'janedoe'},
         httpStatus: 200,
-        req: {name: 'UltramidTKR4355G7_2', supplier: 'BASF', group: 'PA6/6T', mineral: 0, glass_fiber: 35, carbon_fiber: 0, numbers: [{color: 'black', number: 5514212901}, {color: 'signalviolet', number: 5514612901}]}
+        req: {name: 'UltramidTKR4355G7_2', supplier: 'BASF', group: 'PA6/6T', mineral: 0, glass_fiber: 35, carbon_fiber: 0, numbers: [{color: 'black', number: '5514212901'}, {color: 'signalviolet', number: '5514612901'}]}
         ,
       }).end((err, res) => {
         if (err) return done(err);
-        should(res.body).be.eql({_id: '100000000000000000000001', name: 'UltramidTKR4355G7_2', supplier: 'BASF', group: 'PA6/6T', mineral: 0, glass_fiber: 35, carbon_fiber: 0, numbers: [{color: 'black', number: 5514212901}, {color: 'signalviolet', number: 5514612901}]});
+        should(res.body).be.eql({_id: '100000000000000000000001', name: 'UltramidTKR4355G7_2', supplier: 'BASF', group: 'PA6/6T', mineral: 0, glass_fiber: 35, carbon_fiber: 0, numbers: [{color: 'black', number: '5514212901'}, {color: 'signalviolet', number: '5514612901'}]});
         MaterialModel.findById('100000000000000000000001').lean().exec((err, data:any) => {
           if (err) return done(err);
           data._id = data._id.toString();
           data.numbers = data.numbers.map(e => {return {color: e.color, number: e.number}});
-          should(data).be.eql({_id: '100000000000000000000001', name: 'UltramidTKR4355G7_2', supplier: 'BASF', group: 'PA6/6T', mineral: '0', glass_fiber: '35', carbon_fiber: '0', numbers: [{color: 'black', number: 5514212901}, {color: 'signalviolet', number: 5514612901}], status: 0, __v: 0});
+          should(data).be.eql({_id: '100000000000000000000001', name: 'UltramidTKR4355G7_2', supplier: 'BASF', group: 'PA6/6T', mineral: '0', glass_fiber: '35', carbon_fiber: '0', numbers: [{color: 'black', number: '5514212901'}, {color: 'signalviolet', number: '5514612901'}], status: 0, __v: 0});
           done();
         });
       });
     });
+    it('accepts a color without number', done => {
+      TestHelper.request(server, done, {
+        method: 'put',
+        url: '/material/100000000000000000000007',
+        auth: {basic: 'janedoe'},
+        httpStatus: 200,
+        req: {numbers: [{color: 'black', number: ''}, {color: 'natural', number: ''}]},
+        res: {_id: '100000000000000000000007', name: 'Ultramid A4H', supplier: 'BASF', group: 'PA66', mineral: 0, glass_fiber: 0, carbon_fiber: 0, numbers: [{color: 'black', number: ''}, {color: 'natural', number: ''}]}
+      });
+    })
     it('rejects already existing material names', done => {
       TestHelper.request(server, done, {
         method: 'put',
@@ -233,20 +251,10 @@ describe('/material', () => {
         url: '/material/100000000000000000000001',
         auth: {basic: 'janedoe'},
         httpStatus: 400,
-        req: {numbers: [{colorxx: 'black', number: 55}]},
+        req: {numbers: [{colorxx: 'black', number: '55'}]},
         res: {status: 'Invalid body format', details: '"numbers[0].color" is required'}
       });
     });
-    it('rejects a wrong color number property', done => {
-      TestHelper.request(server, done, {
-        method: 'put',
-        url: '/material/100000000000000000000001',
-        auth: {basic: 'janedoe'},
-        httpStatus: 400,
-        req: {numbers: [{color: 'black', number: 'xxx'}]},
-        res: {status: 'Invalid body format', details: '"numbers[0].number" must be a number'}
-      });
-    });
     it('rejects an invalid id', done => {
       TestHelper.request(server, done, {
         method: 'put',
@@ -307,7 +315,7 @@ describe('/material', () => {
           if (err) return done(err);
           data._id = data._id.toString();
           data.numbers = data.numbers.map(e => {return {color: e.color, number: e.number}});
-          should(data).be.eql({_id: '100000000000000000000002', name: 'Ultramid T KR 4355 G7', supplier: 'BASF', group: 'PA6/6T', mineral: 0, glass_fiber: 35, carbon_fiber: 0, numbers: [{color: 'black', number: 5514212901}, {color: 'signalviolet', number: 5514612901}], status: -1, __v: 0}
+          should(data).be.eql({_id: '100000000000000000000002', name: 'Ultramid T KR 4355 G7', supplier: 'BASF', group: 'PA6/6T', mineral: 0, glass_fiber: 35, carbon_fiber: 0, numbers: [{color: 'black', number: '5514212901'}, {color: 'signalviolet', number: '5514612901'}], status: -1, __v: 0}
           );
           done();
         });
@@ -370,7 +378,7 @@ describe('/material', () => {
         url: '/material/new',
         auth: {basic: 'janedoe'},
         httpStatus: 200,
-        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: 5515798402}]}
+        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: '5515798402'}]}
       }).end((err, res) => {
         if (err) return done (err);
         should(res.body).have.only.keys('_id', 'name', 'supplier', 'group', 'mineral', 'glass_fiber', 'carbon_fiber', 'numbers');
@@ -384,7 +392,7 @@ describe('/material', () => {
         should(res.body.numbers).matchEach(number => {
           should(number).have.only.keys('color', 'number');
           should(number).have.property('color', 'black');
-          should(number).have.property('number', 5515798402);
+          should(number).have.property('number', '5515798402');
         });
         done();
       });
@@ -415,13 +423,52 @@ describe('/material', () => {
         });
       });
     });
+    it('accepts a color without number', done => {
+      TestHelper.request(server, done, {
+        method: 'post',
+        url: '/material/new',
+        auth: {basic: 'janedoe'},
+        httpStatus: 200,
+        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: ''}]}
+      }).end((err, res) => {
+        if (err) return done (err);
+        should(res.body).have.only.keys('_id', 'name', 'supplier', 'group', 'mineral', 'glass_fiber', 'carbon_fiber', 'numbers');
+        should(res.body).have.property('_id').be.type('string');
+        should(res.body).have.property('name', 'Crastin CE 2510');
+        should(res.body).have.property('supplier', 'Du Pont');
+        should(res.body).have.property('group', 'PBT');
+        should(res.body).have.property('mineral', 0);
+        should(res.body).have.property('glass_fiber', 30);
+        should(res.body).have.property('carbon_fiber', 0);
+        should(res.body.numbers).matchEach(number => {
+          should(number).have.only.keys('color', 'number');
+          should(number).have.property('color', 'black');
+          should(number).have.property('number', '');
+        });
+        MaterialModel.find({name: 'Crastin CE 2510'}).lean().exec((err, data: any) => {
+          if (err) return done (err);
+          should(data).have.lengthOf(1);
+          should(data[0]).have.only.keys('_id', 'name', 'supplier', 'group', 'mineral', 'glass_fiber', 'carbon_fiber', 'numbers', 'status', '__v');
+          should(data[0]).have.property('_id');
+          should(data[0]).have.property('name', 'Crastin CE 2510');
+          should(data[0]).have.property('supplier', 'Du Pont');
+          should(data[0]).have.property('group', 'PBT');
+          should(data[0]).have.property('mineral', '0');
+          should(data[0]).have.property('glass_fiber', '30');
+          should(data[0]).have.property('carbon_fiber', '0');
+          should(data[0]).have.property('status', 0);
+          should(_.omit(data[0].numbers[0], '_id')).be.eql({color: 'black', number: ''});
+          done();
+        });
+      });
+    });
     it('rejects already existing material names', done => {
       TestHelper.request(server, done, {
         method: 'post',
         url: '/material/new',
         auth: {basic: 'janedoe'},
         httpStatus: 400,
-        req: {name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: 5514263423}]},
+        req: {name: 'Stanyl TW 200 F8', supplier: 'DSM', group: 'PA46', mineral: 0, glass_fiber: 40, carbon_fiber: 0, numbers: [{color: 'black', number: '5514263423'}]},
         res: {status: 'Material name already taken'}
       });
     });
@@ -431,7 +478,7 @@ describe('/material', () => {
         url: '/material/new',
         auth: {basic: 'janedoe'},
         httpStatus: 400,
-        req: {supplier: 'Du Pont', group: 'PBT', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: 5515798402}]},
+        req: {supplier: 'Du Pont', group: 'PBT', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: '5515798402'}]},
         res: {status: 'Invalid body format', details: '"name" is required'}
       });
     });
@@ -441,7 +488,7 @@ describe('/material', () => {
         url: '/material/new',
         auth: {basic: 'janedoe'},
         httpStatus: 400,
-        req: {name: 'Crastin CE 2510', group: 'PBT', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: 5515798402}]},
+        req: {name: 'Crastin CE 2510', group: 'PBT', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: '5515798402'}]},
         res: {status: 'Invalid body format', details: '"supplier" is required'}
       });
     });
@@ -451,7 +498,7 @@ describe('/material', () => {
         url: '/material/new',
         auth: {basic: 'janedoe'},
         httpStatus: 400,
-        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: 5515798402}]},
+        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: '5515798402'}]},
         res: {status: 'Invalid body format', details: '"group" is required'}
       });
     });
@@ -461,7 +508,7 @@ describe('/material', () => {
         url: '/material/new',
         auth: {basic: 'janedoe'},
         httpStatus: 400,
-        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: 5515798402}]},
+        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', glass_fiber: 30, carbon_fiber: 0, numbers: [{color: 'black', number: '5515798402'}]},
         res: {status: 'Invalid body format', details: '"mineral" is required'}
       });
     });
@@ -471,7 +518,7 @@ describe('/material', () => {
         url: '/material/new',
         auth: {basic: 'janedoe'},
         httpStatus: 400,
-        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', mineral: 0, carbon_fiber: 0, numbers: [{color: 'black', number: 5515798402}]},
+        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', mineral: 0, carbon_fiber: 0, numbers: [{color: 'black', number: '5515798402'}]},
         res: {status: 'Invalid body format', details: '"glass_fiber" is required'}
       });
     });
@@ -481,7 +528,7 @@ describe('/material', () => {
         url: '/material/new',
         auth: {basic: 'janedoe'},
         httpStatus: 400,
-        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', mineral: 0, glass_fiber: 30, numbers: [{color: 'black', number: 5515798402}]},
+        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', mineral: 0, glass_fiber: 30, numbers: [{color: 'black', number: '5515798402'}]},
         res: {status: 'Invalid body format', details: '"carbon_fiber" is required'}
       });
     });
@@ -501,7 +548,7 @@ describe('/material', () => {
         url: '/material/new',
         auth: {basic: 'janedoe'},
         httpStatus: 400,
-        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{number: 5515798402}]},
+        req: {name: 'Crastin CE 2510', supplier: 'Du Pont', group: 'PBT', mineral: 0, glass_fiber: 30, carbon_fiber: 0, numbers: [{number: '5515798402'}]},
         res: {status: 'Invalid body format', details: '"numbers[0].color" is required'}
       });
     });
diff --git a/src/routes/validate/material.ts b/src/routes/validate/material.ts
index c8b6e91..b09c494 100644
--- a/src/routes/validate/material.ts
+++ b/src/routes/validate/material.ts
@@ -33,8 +33,9 @@ export default class MaterialValidate {  // validate input for material
         color: joi.string()
           .max(128)
           .required(),
-        number: joi.number()
-          .min(0)
+        number: joi.string()
+          .max(128)
+          .allow('')
           .required()
       }))
   };
diff --git a/src/test/db.json b/src/test/db.json
index 62d18ea..fcd1631 100644
--- a/src/test/db.json
+++ b/src/test/db.json
@@ -121,11 +121,11 @@
         "numbers": [
           {
             "color": "black",
-            "number": 5514263423
+            "number": "5514263423"
           },
           {
             "color": "natural",
-            "number": 5514263422
+            "number": "5514263422"
           }
         ],
         "status": 10,
@@ -142,11 +142,11 @@
         "numbers": [
           {
             "color": "black",
-            "number": 5514212901
+            "number": "5514212901"
           },
           {
             "color": "signalviolet",
-            "number": 5514612901
+            "number": "5514612901"
           }
         ],
         "status": 10,
@@ -176,7 +176,7 @@
         "numbers": [
           {
             "color": "black",
-            "number": 5513933405
+            "number": "5513933405"
           }
         ],
         "status": 10,
@@ -193,7 +193,7 @@
         "numbers": [
           {
             "color": "black",
-            "number": 5514262406
+            "number": "5514262406"
           }
         ],
         "status": 10,
@@ -210,11 +210,28 @@
         "numbers": [
           {
             "color": "natural",
-            "number": 10000000
+            "number": "10000000"
           }
         ],
         "status": -1,
         "__v": 0
+      },
+      {
+        "_id": {"$oid":"100000000000000000000007"},
+        "name": "Ultramid A4H",
+        "supplier": "BASF",
+        "group": "PA66",
+        "mineral": 0,
+        "glass_fiber": 0,
+        "carbon_fiber": 0,
+        "numbers": [
+          {
+            "color": "black",
+            "number": ""
+          }
+        ],
+        "status": 10,
+        "__v": 0
       }
     ],
     "conditions": [